Data: I'm an user of Conch-L where I found the address for your Shell Id page. A friend of mine sent to me the photos of a large freshwater bivalve inhabiting the muddy bottoms of a wide pool where operates some water scooping machines some kilometers north off Livorno, Tuscany, Italy.
I immediately tried with Unionids, but all of them I know from Italy are much more elongate than the the one you can see in the photos.
Thanks for your time
Enzo Campani
Co-ordinator of the
Gruppo Malacologico Livornese
C.so Mazzini 299
57126 Livorno
Italy
Phone +390586803169
Web: http://xoomer.virgilio.it/myzar80/Index.html

Email: Enzo

Identified: Sinanodonta woodiana (Lea, 1834)

Discussion:

• Maybe Sinanodonta woodiana..Lubosh

• your bivalve is Sinanodonta woodiana (Lea, 1834). It ia allready knowm from Italy.

• See for Italy:
  FABBRI, R. & L. LANDI (1999): Nouve segnalazioni di molluschi, crostacei e pesci esotici in Emilia Romagna e prima segnalazioni di Corbicula fluminea in Italia. Quad. Studi Nat. Romagna 19: 9 20
  and for Austria:
  FISCHER, W., A. REISCHÜTZ & P.L. REISCHÜTZ (2002): Die Perschling ein Juwel in einer eintönigen Kulturlandschaft (Niederösterreich). (Beiträge zur Kenntnis der Molluskenfauna Niederösterreich, 19) - Club Conchylia Informationen 33 (4/6): 9-14.
  best regards...Wolfgang