Corbicula sp. (at least 2 are present in the U.S. as invasives and a
third invasive elsewhere; the species have hybrid origins making
genetic analyses confusing and the Asian native forms need thorough
review to establish what are valid names that might apply to the
invasives) differ from native extant freshwater bivalves in having
strong commarginal ("concentric") sculpture.  The primarily
brackish-water Polymesoda is a native member of the same family.
Young and some older individuals are light yellow; older individuals
are often dark brown.  Fingernail clams (Sphaerioidea) are much
smaller as adults, with generally weak sculpture.  Most adult unionids
are larger.  All unionids have pearly rather than porcellaneous inner
shell layers and lack the distinct cardinal and lateral teeth of
Corbicula; unionids are either edentulous or have teeth not readily
classified as cardinal versus lateral.

--
Dr. David Campbell
425 Scientific Collections
University of Alabama
"I think of my happy condition, surrounded by acres of clams"

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