Potamilus fragilis shell description:

“Shell large, thin, obovate, subcompressed to subinflated, with generally low compressed beaks having very feeble sculpture, which shows a tendency to be doubly looped; posterior ridge almost wanting, there being two or sometimes three radial raised lines on the posterior slope; there is a moderately developed posterior wing, which is broken away in adult specimens showing the long ligament, and in front of the hinge the young shell is angular; surface rather smooth, with faint, irregular growth lines, greenish-yellow or pale smoky-brownish, sometimes feebly rayed, often rayless, the posterior slope dark green and generally rayed; left valve with two feeble, compressed pseudocardinals and two remote, often imperfect, laterals; right valve with one pseudocardinal and one truncate lateral; beak cavities shallow, showing a row of ill-developed muscle scars running in the direction of the retractor muscle scar; adductor scars large, faint, the anterior irregular; nacre faint purplish and bluish.  Generally the male and female are much alike, the former is sometimes a little rhomboid and again it ends in a wide, rounded point about on the median line.  The female shell is a little fuller and more rounded on the post-basal region, and sometimes has a well-developed marsupial swelling ”(Simpson, 2014; Mather, 2007).