Pisidium casertanum, the pea cockle or pea clam, is a minute freshwater bivalve mollusc of the family Sphaeriidae.
The shell is broad, sub-triangular or oval and is ornamented with sculpture of faint concentric striations. The umbos are slightly behind the middle. The Periostracum is silky, scarcely glossy. In colour it is whitish to grey-brown and often the shell is coated with reddish-brown deposits.
The shell is of similar shape to Sphaerium novaezelandiae but is smaller as an adult, more inflated, with a deeper hinge-plate, stronger teeth, and the ligament is not visible externally.
Length is up to 4.5 mm, height 3.7 mm, and thickness 2.3 mm.
It has a cosmopolitan distribution and is perhaps the world's widely distributed non-marine mollusc.
