>Obviously, mollusks entered fresh water more than once (as snails and clams,
>at the very least). How many times did it actually happen?

In the Bivalvia, multiple times:
unionoids, sphaeriids, corbulids (possibly multiple invasions),
dreissenids, lymnocardiids, Scaphula (Arcidae), myalinids (Paleozoic),
palaeomutelids (Paleozoic), Cyrenoididae, Novaculininae (Pharidae), and
Ostomya and relatives (Lyonsiidae), plus several Paleozoic groups of
uncertain antecedents.  Probably others exist which I do not remember.  A
problem lies in the distinction between estuarine or brackish and truly
freshwater species.

Several distinct groups of snails are freshwater as well, including
neritoideans, caenogastropods, opisthobranchs, and pulmonates.


Dr. David Campbell
"Old Seashells"
Department of Geological Sciences
CB 3315 Mitchell Hall
University of North Carolina at Chapel Hill
Chapel Hill NC 27599-3315 USA
[log in to unmask], 919-962-0685, FAX 919-966-4519

"He had discovered an unknown bivalve, forming a new genus"-E. A. Poe, The
Gold Bug