Taxonomy of the Phylum Mollusca
by Winston Barney, June, 2000

How Scientists Classify Mollusks

     Classification is the arrangement of organisms into groups with regard to their natural relationships (systematics) and according to established criteria (taxonomy). The terms classification, systematics, and taxonomy are often used interchangeably because they apply to the same concept of arranging things in their "correct" relationship.

     We classify things as soon as begin to think... as toddlers we soon learn the different colors and shapes. We develop tastes for "good" food and "bad" food, and learn the difference between parents, friends, and "strangers". In our adult lives we classify many items in our daily routines: department stores are, by definition, divided into different departments. Grocery stores have separate sections for different kinds of items. Auto dealers, electronic stores, fast food chains are all categorized. Most children can sort a pile of seashells into fairly correct relationships simply by comparing shapes. But we know that the relationships between living things go much deeper than shape or size, so let us examine the ways that scientists classify the subject of this site - the molluscs!

     The following Classification Hierarchy chart is a visual aid to understanding how scientists classify living things. Follow the chart from left to right and down. There are seven major categories: Kingdom, Phylum, Class, Order, Family, Genus, and Species (NOTE: You can make up sentences to remember the order (called mnemonics) - how about "King Phillip Comes Over For Greasy Sandwiches"?).   Each level of classification can be divided into as many of the next lower level as needed - for example, some families might have only a single genus (the Family Conidae (the Cone Shells) has only the single genus Conus, while the closely-related Turridae are divided into anywhere between 200 and 300 genera (and about 15 sub-families!), depending upon which authority you believe!!) . The categories beginning with the prefixes "super-" and "sub-" may be used to further subdivide groups when needed. (in the Phylum Mollusca, the level of "superfamily" is particularly important: Cones and Turrids, for example, are in the same superfamily.)

Kingdom /                        (Subclass)
              \               Class /                                    Family
              Phylum /                       (Suborder) /
                            \           Order /
                            Class /                                          Family                        Species
                                     \             (Superfamily) /                      Genus /
                                     Order /                                    (Tribe) /
                                                \              (Subfamily) /
                                                Family /                                Species
                                                             \              (Subgenus) /
                                                           Genus /
                                                                       \              (Subspecies)
                                                                       Species /
                                                                                                              (Varieties and Forms)


     The road to classifying a living organism begins with its placement in a kingdom. There are five kingdoms of living things: Animalia (animals), Plantae (plants), Protista (one-celled organisms), Procaryotae (bacteria and blue-green algae) and Fungi (plural of fungus) (then there are Viruses, which are semi-alive, since they cannot reproduce by themselves, "Viroids", which are mini-viruses, and something called "ricketstae" (or something close!!)).  Molluscs are animals so they belong to the animal kingdom.


     Within the animal kingdom there are two sub-kingdoms or branches. They are the Invertebrata (invertebrates), which have no backbone , and the Vertebrata (vertebrates), which have a backbone (We are mammals, which are vertebrates of the family Chordata, class mammalia). The number of living invertebrate species is over 800,000 (possibly up to 20,000,000! (most experts concede at least 10 million), and the vertebrates number around 70,000 species.


     The invertebrate subkingdom contains many groups called phyla. Examples of some familiar phyla are the Protozoa (protozoans), the Porifera (sponges), the Coelenterata (jellyfish, anemones, and corals ), the Platyhelminthes (flat worms), the Nemathelminthes (round worms ), the Annelida (segmented worms), the Echindodermata (starfish, sea urchins, sand dollars ), the Mollusca (snails, slugs, clams, oysters, scallops, octopuses, squids ), and the largest phylum ("Phylum" is the singular of "Phyla") by a long shot, the Arthropoda (insects and crayfish).

    The name Mollusca  comes from the Latin word molluscus meaning soft. The phylum Mollusca is made up of animals that have no backbone and possess  internal cavities containing the heart, kidneys and gonads. Most molluscs have a hard exterior shell that is produced by the mantle, a fleshy fold of the body, however some, such as the slugs, have no shell at all. Some molluscs (squid and seahares (Pteropods) produce an internal shell.


     The division of the phylum Mollusca into smaller groups is based on major morphological (form, especially that of the shell) and anatomical  features (i.e., internal organs) that differentiate each class from the others. Here are the ways some of the early naturalists attempted to define the classes of the phylum Mollusca:

       Aristotle (384-322  B.C.) divided the mollusca into two groups:
         (1) Malachia  (present day cephalopods)

         (2) Ostracodermata (all shelled molluscs, further divided into univalves and                bivalves)

      Pliny ( the Elder, 23-79 A.D.)
         (1) changed the name Malachia to Mollia

         (2) changed the name Ostracodermata to Testacea

       Jonstonus (1650) changed the name Mollia to Mollusca

       Linnaeus (1758)
         (1) used the term Mollusca to describe a large number of soft animals including slugs, tunicates, squids, octopuses, anemones, star fish, sea urchins, pteropods, jellyfish, and marine worms. (NOTE: Linnaeus was primarily a botanist (i.e., he studied plants), but wanted to name and classify all the living things in the world, so he tackled everything, even when he perhaps shouldn't have!!  He is the "father of modern taxonomy", inventing the basis of the modern system by which each species is given a "binomial" name, consisting of a genus (which can change!), and a species (which usually stays the same: the first name used to describe a species takes precedence, so if a species (say, Hydrobia totteni Morrison, 1954 (a small salt-marsh species found in New England and SE Canada) is found to have been described earlier by another author (in this case H. truncata Vanatta, 1924), then the species must be called by the first known name (in this case, all the museums would have to change the name from H. totteni to H. truncata). The system he proposed, took over 100 years to catch on virtually universally, but now we have a completely worldwide set of rules and guidelines, and a ruling body called the ICZN to make taxonomic decisions and rulings - briefly discussed below.

         (2) used the term Testacea to describe gastropods, bivalves, chitons,               nautiloid cephalopods and barnacles.

      Cuvier (1795)  subdivided molluscs into:
         (1)  Cephalopodes

         (2)  Gasteropodes ( including slugs without shells )

         (3)  Acephales ( bivalves, tunicates, brachipods, and barnacles )

     Dumeril (1806)  divided molluscs into:
         (1)  Cephalopodes

         (2)  Pteryopodes (pelagic molluscs who move by flapping their parapodia)

         (3)  Gasteropodes

         (4)  Brachiopodes (including barnacles)

         (5)  Acephales ( including tunicates)

     Lamarck (1801) divided molluscs into:
         (1)  Cephales ( Cephalopodes, Gasteropodes, and Pterypodes)

         (2)  Acephales ( including brachipods)

     Lamarck (1815-1819) removed bivalves and brachipods from the phylum so      that mollusca now included:
         (1)  Pteropodes

         (2)  Gasteropodes

         (3)  Trachelipodes ( with a spiral shell)

         (4)  Cephalopodes

         (5)  Heteropodes (another group of pelagic molluscs)

     By now, the reader has realized that classification is not a static thing, but an evolving process, changing with our increasing knowledge, and new theories of how organisms are related to each other. The early naturalists were trying to contribute to the body of knowledge using only the specimens on hand and very little prior literature for comparison. During the nineteenth century scientific voyages were undertaken for the sole purpose of collecting species of all phyla from both the animal and the plant kingdom. . The species collected were often immediately transferred to the cabinets of wealthy collectors and the jobs of describing and naming of these species were  undertaken later.

     During this time naturalists described species from widely disparate areas of the world. At times the same species was given two different names by two different authors. This happened when an author discovered and named a new species not knowing that another author, in another place, had already described it. (see note on Linnaeus, above.  One of the most extreme examples is Conus mediterraneus Hwass, 1792 (the only cone in the Mediterranean Sea), which was given over 120 names by different people over the years!!  All of these names are now "synonyms" of the original name - i.e., they refer to the same thing!) Compare below some of the problems the early workers encountered and note how they are being solved in the twentieth century.

I.  Methods

THEN: Early collectors depended on morphological characters of shells,  and gross anatomy to classify  their molluscan species.
NOW:  Modern technology has infinitely expanded our capabilities to analyze a specimen.  Now, we use detailed anatomy,x-rays, electron microscopes, biochemistry, DNA comparisons, ecology, behavioral studies, and even fossil histories to find relationships previously undetectable.

II. Conformity

THEN: A very debilitating aspect was the lack of a universal plan to be obeyed by all when  naming new species. Although most followed Linnaeus' lead in giving species a two-part (binomial) name, there was little agreement regarding the proper rules for naming new species, until very recently.
NOW : To resolve conflicts the International Code of Zoological Nomenclature was written   (the first edition published in 1961) to set the rules for naming (or renaming) taxa. A  taxon is a scientific name for any of the groups in the classification hierarchy. (as in Kingdom, Subkingdom, Phylum, Class, Order, Superfamily, Family, Genus, Species....these are all taxa (the plural of taxon))

III. Communication

THEN: One of the greatest obstacles to scientific agreement was lack of communication among scientists in all parts of the world..
NOW:  Today we have  international travel and email to bring us together within hours or even    instantly. Scanners and digital cameras allow us to send and receive information and  pictures at will. Universities store wonderful libraries of monographs and and iconagraphies. Museums contain large collections of species for comparison.

Today we recognize these seven classes of molluscs:

(1) GASTROPODA  (gastropods). The name means "stomach-footed".

     These are animals with a well developed head bearing eyes and tentacles, The foot is usually large and muscular and is especially useful for crawling. Most of the gastropods produce an outer shell which is spirally wound, but this class also includes the slugs which have no shell. A unique characteristic of the gastropods is a process called torsion, in which the immature  bilaterally symmetrical animal twists itself into an asymmetrical shape, becoming spirally coiled.

(2) BIVALVIA (bivalves), also called PELECYPODA. The names mean "two-valved" and  "hatchet-footed" respectively.  An older name for the class was LAMELLIBRANCHIATA.

   These animals produce two shells (valves) that are connected by a hinge. The animals have no  head but with a foot that often extends between the two valves.  Common names for  some of the bivalves are clams, scallops, and oysters.

(3) SCAPHOPODA (scaphopods). The name means "boat-footed"

   Animals without eyes, gills, or tentacles. They are enclosed in tusk-shaped shells, open at each end, which gives them their common name, tusk shells.

(4) APLACOPHORA (solenogasters). The name means "without plates" (without shell).

   Worm-like animals having no shell . The animal has no head, mantle or nephridia (excretory organs). The foot has been reduced to a short ridge down the middle of the animal. Instead of a   shell the animal is covered with tiny calcareous spicules.

(5) POLYPLACOPHORA (chitons). The name means "bearing many plates".

   These are bilaterally symmetrical , flat molluscs with a shell consisting of eight overlapping plates  called valves, which are attached underneath and around the edges to a tough, flexible part of  the mantle called the girdle.  The head has no eyes or tentacles. The animal has a radula  designed for heavy-duty rasping.

(6) MONOPLACOPHORA (gastroverms and segmented limpets ). The name means "bearing one plate" (bearing one shell).

   Bilaterally symmetrical cap-shaped shells. The animal is without torsion, but with head, foot, and mantle, and possessing a radula. The muscles, external gills, nerve branches, and nephridia are paired.

(7) CEPHALOPODA (cephalopods) The name means "head-footed".

   This class is characterized by animals with distinct heads and large, complex eyes. The foot has been altered into a set of arms that circle the mouth and parrot-like jaws ("beak"). Except for the chambered nautilus, the molluscs of this class do not have an external shell. Common names for cephalopods are octopus, squid, cuttlefish, and nautilus.

NOTE: For a much more detailed, systematic account of each Class, see the Advanced Introduction to the Mollusca Article.


     The subclasses are exceedingly important divisions in molluscan classification because they represent very significant differences among the animals in each class.

 Gastropoda was until recently (about 15 years ago),  divided into four subclasses, as follows.  As soon as i look up the diagnostic charactaristics of the more recent divisions, i will also post them!
  (1) Prosobranchia (anterior gills (i.e., near the head); gills in front of the heart).

  (2) Opisthobranchia (posterior (i.e., towards the back, away from the head) gills;        gills located to the right and behind the heart).

  (3) Gymnomorpha (bare form; no shells)

  (4) Pulmonata (with lungs rather than gills).

 Bivalvia is divided into five subclasses:
  (1) Protobranchia (first-gilled; among the first molluscs to have gills).

       This is a  new name for the combined groups Paleotaxodonta and        Cryptodonta.

  (2) Pteriomorphia (wing-shaped)

  (3) Paleoheterodonta (ancient differentiated teeth)

  (4) Heterodonta  (differentiated teeth; with distinct cardinal and lateral teeth)

  (5) Anomalodesmata (irregular binding; absence of true hinge teeth)

  (1) Chaetodermomorpha (long hair-skinned form; the skin consists of long        spicules)

  (2) Solenogastres (channeled stomach)

  (1) Coleoidea (like a sheath or scrotum; the animal has no outer shell)

  (2) Nautiloidea,  with chambered shells.

 Scaphopoda, Polyplacophora, and Monoplacophora are not divided into subclasses by most taxonomists.


Every class of shells contains one or more orders. Each order identifies an important characteristic that is common to all the molluscs included in it. Orders are significant groups in the classification process and provide another step in the precise classification of a species.

 Gastropoda contains 17 orders and 11 suborders.
 Bivalvia has 12 orders and 5 suborders.

 Scaphopoda is divided into 2 orders.

 Aplacophora contains 4 orders

 Polyplacophora has 1 order with 3 suborders

 Monoplacophora has 1 order

 Cephalopoda contains 5 orders and 3 suborders.

All the names of orders have the ending "-OIDEA". Example: STROMBOIDEA


     Referring to the chart you will see that each class can be divided into several families. A family is a level of classification that consists of animals that share all the characteristics of their class, but differ from each other in other ways. For example, the families Olividae (olives) and Muricidae (murexes) both belong to the class Gastropoda, but they are instantly differentiated by their appearance, olive shells being smooth and glossy while murex shells are mostly covered with spines or leaf-like constructions.

It is customary for all the names of families to have the ending "-idae". Example: Strombidae

    Here are some of the other characteristics used to determine placement of molluscs into orders, families, and genera:

All molluscs:

Structure of the gills, heart, and nervous system
General outline, shape, and size of  shell (oval, conical, biconical, auriform,    pyriform, trochiform, turriform, turbinate, fusiform, irregular, circular, quadrate,    flabelliform, cardioid, patelliform, etc.)

General sculpture features (smooth, corded, scabrous, spiny, pustulose, foliaceous, etc.)

Gastropods: (NOTE) See the article Advanced Introduction to the Mollusca, for definitions and explanations of many of these terms - but you can look them up in any good Scientific Dictionary!

Conus marmoreus

Chicoreus ramosus

Strombus gigas

Helix pomata
  •    Structure of the radula (rhipidoglossan, docoglossan, taenioglossan,    ptenoglossan, stenoglossan, and toxoglossan)
  •    Structure of the operculum (chitinous, calcareous, multispiral,    paucispiral,    ungulate, ovate).
  •    Sculpture, color, texture and number of whorls in the protoconch
  •    Character of the aperture
  •    Character of the columella (curvature, sculpture)
  •    Presence and character of umbilicus






Spondylus americanus
  •    Possession of a byssus (used for attaching to the bottom, or other    substrates)
  •    Structure of the hinge (taxodont, dysodont, isodont, heterodont)
  •    Structure of the ligament (alinvicular,    transverse,duplivincular,multivincular)
  •    Number, position, size, and purpose of the muscle scars    (monomyarian,    heteromyrarian,  homomyarian, dimyarian,  adductor, pedal retractor, byssus    retractor)
  •    Form of pallial line (formed by the position of the mantle inside the    shell) (    integropalliate; entire , sinopalliate; with sinus )





Miscellaneous tusk shells
  •    Differences in the radula
  •    Cross section of shell (quadrate, circular, polygonal)
  •    Shape of posterior end (apical notch or terminal siphon)
  •    Presence, number, and shape of longitudinal sculpture (sculpture    refers to    the fine details on the surface of the shell - they can be ribs,    scales, ridges,    etc. The term "micro-sculpture" refers to structures    that can only be seen    easily when magnified)







  •    Presence of longitudinal ventral groove
  •    Presence and character of  calcareous spicules (remnants of a shell)
  •    Character of mouth
  •    Presence and character of radula


  •    Sculpture of the plates (granulated, radial ribs, longitudinal ribs)
  •    Details of plate design (articulation, division into regions, notching at edges)
  •    Texture of the girdle
  •    Presence of bristle tufts




  •    Number of paired gills
  •    Sculpture of shell
  •    Projection of apex in relation to shell margin





  •    Number and size of arms (eight or ten)
  •    Presence of web connecting the arms
  •    Character of suckers (stalked or unstalked)
  •    Presence of internal shell
  •    Presence of external shell
  •    Occurrence of sexual dimorphism
  •    Presence of a hectocotylus
  •    Presence of  ink sac
  •    Presence of luminescent organs
  •    Ability to change colors


     At the generic level (genus) the animals are sorted into even smaller, but more closely related, groups. This is the first part of the scientific name is given is given to a species - the first part of its two-part name (binomen), by which it will always be known. (example: Homo sapiens: The genus is Homo).
NOTE: This name is given in the ancient language of Latin, which up until very recently, was the language that scientists and other scholars all over the world used to communicate with each other (if you went to university, you almost automatically learned Latin, so you could read scientific literature and communicate with scholars from other countries). Today, there is no such universal language, although English is starting to assume this role.

     Characters used to place an animal in a genus are even finer details than those used to place it in a family. For example, the type of radula it possesses may place it in a particular family, but the actual shape and count of the central, lateral, and marginal teeth place it in a genus with other animals having the exact same structure. By the same token in bivalves, though the hinge structure  may place the animal in a particular family, the shell shape or sculpture may distinguish it from other genera in the family.


    The "final" step of the classification hierarchy is the specific (species) level. At this point scientists have developed a definition that applies to all  living organisms. Although authorities do not  agree on the exact wording of the definition, the consensus is that a species must be a population of individuals that interbreed and are able to reproduce fertile offspring only when breeding with their own species
(NOTE: "hybrids" are formed when two closely-related species interbreed (eg: a mule is the offspring of a horse and a donkey).  They are sterile, meaning they cannot have offspring to continue their line) This is perhaps the most important level of classification because it defines an organism that can not be like any other organism in the world.  Some scientists estimate that there may be up to 100 million species of life on our little planet (only about 2 million are known to science so far!!), while others put the number at as low as 5 to 10 million species sharing "Spaceship Earth" with us.
     Obviously this requirement is hard to observe except for animals in a carefully monitored environment, that is, we are not always able to witness and document the reproductive process. Thus we often have to depend on physical comparisons of the animal and shell to validate that an animal belongs to an assigned species.

    The animal is now given the second part of its two-part name. From then on the animal is called by its scientific name which consists of its genus name followed by its specific name. For example, the scientific name
Strombus marginatus indicates that the species marginatus is a member of the genus Strombus.
In scientific literature, the genus, species, subspecies and form/variety names are usually italicized or otherwise specially designated,  while taxa above the genus level (family, superfamily, order, class, etc.) are merely capitalized.  In formal scientific literature, the name of the person who described the species (called the Author, since he or she is the author of the paper or book that the species was first described (which is how species are named: you "describe" how a given species is different from all other species), and the date of the publication where he or she described it.  Thus, the full name would be Strombus marginatus
Linnaeus, 1758, since it was first described in Linnaeus' multi-volume work called Systemae Naturale, where he set out to describe all living things known at that time - an ambitious task!!  In more informal literature, such as this article, often only the author's name is given.


     A subspecies is an animal that obviously belongs to a particular species except that it has developed one or more specific characters that are different from the parent group. It is geographically separated from the parent group (i.e., it is separated by geographic features such as mountains, land, etc.) but may produce intergrades where their ranges overlap. In this case the animal may be given a subspecific (subspecies) name. For example, Strombus marginatus from the Philippines varies in shape from the typical Strombus marginatus of the India, so it is given a name consisting of the genus, species and subspecies monikers. This arrangement is called a  "trinomen". The Philippines subspecies is named Strombus marginatus septimus.

The complete classification of this animal is therefore:
              Kingdom: Animalia

                           Phylum: Mollusca

                                Class:    Gastropoda

                                     Order:   Prosobrachia

                                          Family:  Strombidae

                                              Genus:   Strombus

                                                   Species:  marginatus

                                                        Subspecies: septimus


      When two animals of the same species produce different shapes, colors or patterns of color with some regularity, these variants are often given a variation name or a form name. For example, the Strombus marginatus from India has two forms. One has a ridge (carina) around the body whorl, the other has no ridge but is considerably rotund. The latter has been named Strombus marginatus forma robustus, whereas the former is called S. m. forma carinata.There are basically  two types of varieties or forms (the terms are often used interchangeably - their usage in the literature is quite inconsistant!!):
1) Ecophenotypes:
 These are either a) forms which have adaptive value in a given habitat condition: for example, in Nova Scotia, the Muricid (i.e., family Muricidae) species Nucella lapillus (Linne, 1758), sometimes occurs in areas where they can land or be washed onto soft, silty bottoms (normally they live on hard substrates, and have limited tolerance for being buried under sediments).  In these areas, a form often develops with "frills" on their bodies, so they don't sink into the silt so easily, where they could smother.  This form is called N. lap.var. imbricata Lamark, 1822, and is only found in low - energy environments: where the species occurs in high-energy environments such as rocks exposed to surf, this form is just not found, or b) non-adaptive changes which occur in certain environmental conditions: a good example of this is "melanism", which is a condition where extra pigmentation occurs in the shell of cowries in particular, in response to high levels of nickel and/or iron in the water where it grows (frequent in some areas of New Caladonia, where some cowries can be completely black from this condition, but early stages of melanism occur in cowrie populations in many parts of the world).  This  response is often given the name "niger".  The other kind of variety or form is called a
2) Genotype: This is where a form is the result of genetics: the most common of these is albinism, where the organism is incapable of procuring pigmentation of any kind.  This occurs to some extent in almost all populations of all species, but in many cases, it is fatal, causing the individual affected to die very young.  Thus, in nature, only some species have specimens which grow to adulthood in an albinistic condition.

    It should be noted that in many cases, it is difficult to determine whether a given variety is the result of "nature" (a genotype), or "nurture" - a response to specific ecological conditions.  It is also often difficult to determine if a given form has any adaptive value.  However, in situations where a lot of different forms occur together in a population sharing the same environmental conditions (as in the case where there are many color & pattern forms in the same population (e.g.: the Pecten (family Pectinidae) Chlamys nobilis Linne, has literally dozens of named color and pattern forms, which all live together in the same population: this means they are probably the result of random genetic drift, and not a valid response to environmental conditions)), it is easy to infer that they are genotypes of no intrinsic adaptive value (hence useless to all but collectors who want to collect a lot of named varieties and forms!!). HOWEVER, in some cases, a genetically-determined (i.e., genotypical) form or variety may breed true, and may be found in a limited range, geographically-speaking.  This occurs quite often in land snails, reaching an extreme in the Caribbean tree-snail Liguus fasciatus Muller, 1776, which has about 200 named forms in Florida and Cuba, some of which occur only on a single small mound of earth (called a "hummock") in the Everglades!

     The I.C.Z.N. (the body which makes and modifies the rules of taxonomy for all life-forms on the planet) gives limited approval for varietal or "forma" names (for example, ecophenotypes are often considered valid, in cases where  they are useful ecophenotypic designations for describing adaptations to specific habitats (like the imbricata form of Nucella lapillus discussed above),  or  true-breeding genotypes like the many varieties Liguus fasciatus ,whereas albino or other color forms are not, since they can occur in almost any population of a given species -  but collectors and seashell dealers find these to be extremely helpful in describing the shells.   Cowrie (Family Cypraeidae) collectors take this to the greatest extreme, for some reason only known to other cowrie collectors.


    When a specimen is found, there are several means of identifying it: 1) you can ask someone who knows what it is - for difficult species, such as those in deep water,  an expert in the taxa (family or genus, usually), or in a particular area (e.g.: an expert in the molluscs of Florida, such as Dr. Harry G. Lee) , or for easy species, just someone who lives in the area it was found ("Oh, i knows what THAT one be: it's a Common Periwinkle, that's what it is for sure!!" - then you look up in a book and see that "common periwinkle" is Littorina litterea  Linne, 1758), or 2) look in a book and compare it to correctly-identified photographs or diagrams of species in its family or the area where it was found, and reading the text of the book when in doubt (this is in fact how most species are identified: it is a matter of a) having the correct literature, and b) being familiar enough with molluscs to notice the details necessary to tell similar species apart (a matter of experience: not always as easy as it may seem, since some species are so variable that they often appear to overlap with other, very similar species!)  However, suppose you are an alien, coming to earth and trying to figure out what name our species has assigned to the specimen you happen to be holding!  In this case, you must painstakingly compare your specimen to the descriptions and diagrams of each taxonomic level, from phylum on down.  Once you determine it is a mollusc (phylum Mollusca), you then tackle the class (the subkingdom level is not really important for identification!!): Does it have  a shell?  Yes?  Does it have one, two or 8 parts to its shell? (if one, it is a Monoplacophoran or Gastropod, if two, it is a Bivalve, if 8, then it is a chiton, or Polyplacophoran.) One part to the shell: is it a deep-water shell shaped like a little cone? (Monoplacophoran) No.  Ok, it must be a gastropod. Do you have the animal? If so, you can work on the subclass by looking at the gills or lungs, then work your way down.  For some particularly difficult taxa, such as some genera of Turridae, you will find the radula quite useful at many levels, including the species level.  If you DON'T have the animal, in practice you must jump right from the class level all the way down to the family level: NOT always easy!!! (although in everyday practice, this is the way those pesky humans normally do it!!)  - You must painstakingly figure out just what defines each family the shell could belong to: how is a Muricid different from a Buccinidae species?  How is one kind of Limpet family different from confusingly similar families?? - humans seem to be able to learn this rather easily, but it is actually quite tricky when you have to try to put it into words: get a bunch of shell collectors together and start asking difficult questions about how to tell similar families and genera apart, and you'll find out that as the alien you are for the purpose of this "thought experiment", it isn't easy at all!!!  So, using all kinds of literature, some of which contradicts other literature, or in the case of the early descriptions, is INCREDIBLY vague at times, you struggle down to genus level: now - what species is it???  You look in books and articles about that genus, and for the area you found the shell, and carefully compare diagrams, drawings, photographs, descriptions in various languages - but with no luck!!!  You  at last think it might be a particular rare species, which is almost impossible to identify just using the literature (believe it - this happens more often than you might think!!).  What now??  You must carefully compare your specimen to the original specimen(s) used in describing the species you think it might be, as well as similar species: these are called "type specimens" (see the article "How to Name a Mollusc" for details on this - it is quite fascinating!!!), and are "always" deposited in internationally-recognized collections: museums, universities, etc. - which means you must get permission to see them, then travel to where they are. OOPS!! - after all that work, you find out it is a new species, unknown to science - i.e., you can't find a type specimen anywhere which looks like it close enough - you must now either give it to an expert to be named (perhaps after yourself - Strombus borgensis (how could anyone refuse?  - resistance is futile!!)), or you must become a scientist and describe and publish it yourself - so you decide Earth has too much biodiversity and you go back to your home planet, defeated by carbon-based life-form taxonomy. (and i DO hope the real author (i.e., Winston) of this article will forgive this whimsical exercise by the editor (Ross) - strange things happen after 3 a. m.!!)

     Authorities still disagree on the systematic arrangement of some families, e.g., Cypraeidae, Muricidae, or Pectinidae. That is why it is important to access the most current literature.  One of the problems of identifying just a shell is that some juvenile shells look nothing like their adult parents. For example, some species of Strombus are shaped like cone shells in the juvenile stage but grow expansive outer lips as adults. When one species has two different forms (dimorphism), it is difficult for the casual collector to determine the correct name if he trying to identify a juvenile. So, before you start to identify a shell without the body, you should ask yourself: Does this look like a fully-developed adult?? (is the lip thick or thin?  Is the shell extremely light and fragile (usually means it is a juvenile) - if you are either our friendly, determined alien above, or a beginner in Conchology (the study of shells), or simply unfamiliar with the taxon the shell belongs to, you might be fooled rather badly, and never get to the correct family, let alone the right species (cowries are the worst! - a juvenile cowrie can challenge even world-class experts!!!)

    BUT, in case you are despairing of ever being able to easily identify shells, 7 times out of 10, it is just a matter of looking at pictures in a book, until you find the name of the species whose representative you are holding (the other 3 are what experts and shell dealers are for!!).

    Even after finding the correct species name, you may still have to make a choice between one classification system or another. Cowry collectors are torn between placing all their prizes in one genus (Cypraea) or in any of several systematic arrangements, the most complicated of which contains 35 genera!!!  Authorities who study one family (often to the exclusion of other families) tend to split it into more genera due to details they discover through extensive comparisons of thousands of specimens. They thus earn the nickname "splitters".  Splitters are also prone to use many form or variety names to identify minor differences between groups of specimens, even when these differences are of the "not useful" kind, as discussed above. Indeed many verbal wars have been waged over the validity of names.    Collectors or researchers who prefer to list fewer genera in each family (often, these are "generalists" - not specializing in one narrow field, but studying or just collecting a variety of families), mainly because the shells are obviously related (as in the case of Cypraea), are labeled as "lumpers".  (NOTE: For a great discussion on the "lumpers vs splitters" debate, see

    In the end, you will have to decide for yourself which way you want to label your specimen, based on your study and your need or desire for correctness: if you are a very casual collector, you could just name your specimen "Sammy the Snail", but if you are a specialist in a particular family, you might go with Strombus marginatus  septimus  forma carinata  (G. B. Adams, 1856) (ok, i just made the date and author up - so sue me!).

     It is inevitable that some scientific names will change as scientists continue to delve into their fossil histories and study their detailed anatomy, reproductive methods, and of course their DNA.  It is also clear that many families have not received the close scrutiny of scientists that the more popular families have experienced. (Turridae, for example, even though it is by far the largest family of molluscs in the world (thousands and thousands of species so far!), is still a taxonomic "wilderness", where you can rather quickly go insane if you aren't quite steely-minded!!) A case in point is the family Marginellidae, which until a few years ago was classified almost completely on shell characters. But a dedicated malacologist Gary Coovert,  and his wife, undertook the job of  analyzing the shells and animals of all species and found many characters that begged the placement of many genera in another family. As a result many species formerly assigned to the Marginella family are now placed in the family Cystiscidae, which most collectors and many museum curators completely ignore, since they know that in 20 years, another system of taxonomic classification will probably come along, with the arrival of easy and quick DNA sequencing and comparison (for example!), and they will just have to change all their labels and records once again!!

     It is evident that the classification of  species is complicated. But the difficulties are eased by the rigid structure of the classification hierarchy. Everything must fit in its niche. One who wishes to pursue the study of classifying molluscs must resolve to read  books , periodicals and internet postings, and to actually use his or her brain rather often, no matter how hard it hurts.  A daily dose of the Conch-L list server (if you've made it this far, you really SHOULD consider joining!! - go to  easy-to-follow directions on how to join this on-line discussion group.) will heighten your enthusiasm and acquaint you with questions, answers and discoveries from personalities throughout the conchological world. For those who wish to get REALLY scientific, the Mollusca internet list is the way to go: follow the easy instructions at  See you there!

     Joining a local shell club and an international group,such as Conchologists of America (C.O.A.) is another way to broaden your knowledge and keep you current.
See the Shell Clubs Page for more information

Happy Shelling To All !

This is a new counter system set up by Globel on
December 01, 2002