24 Trans. Acad. Sci. of St. Louis. 



follows, drawn from a large series of specimens from the Des 

 Moines river, Iowa, and supplemented by very large numbers 

 from nearly every considerable stream within the geographic 

 range of this form. 



Shell large, elliptical, compressed, rounded before, sub- 

 biaugulate behind, smooth or striate, thickened anteriorly, 

 thin and iridescent posteriorly; epidermis yellowish-straw 

 color, rayed with numerous, broad, green rays, extending 

 from the umbones ventrad ; the rays are indistinct or wanting 

 anteriorly; lines of growth numerous and often, especially in 

 old specimens, raised into ridges which are concentric with 

 the ventral margin ; ligament long, thick, black, nearly 

 straight ; umbones scarcely prominent, approximating, with 

 many very fine, concentric folds, apparent only in young 

 specimens with perfect epidermis ; from the posterior edge of 

 the umbones an obtuse angle extends over the disk poste- 

 riorly to the margin where it is apparent at one of the angles 

 which renders the outline biangulate; cardinal teeth double 

 in the left, and disposed to be double in the right valve, trian- 

 gular, crenulate, roughened ; lateral teeth long, lamellar, 

 slightly curved ventrad, crenulate ; plate between cardinal 

 and lateral teeth incrassate, arched, smooth ; anterior cica- 

 trices large, deeply impressed, distinct ; posterior cicatrices 

 large, slightly impressed, confluent, that of the retractor pedis 

 muscle impressed at the extreme end of the lateral teeth ; 

 dorsal cicatrices in the cavity of the umbones as deep pits dis- 

 posed in a straight line, which ends near the margin of the 

 plate ; pallial cicatrix crenulate, deeply impressed anteriorly ; 

 nacre pure white, iridescent, in many specimens with a blush 

 of pink or with decided pink coloration. 



Length, 125 ram. ; height, 75 mm.; breadth, 52 mm. 



The measurements given are those of a large specimen 

 from the Des Moines river, at Des Moines. The species often 

 exceeds these dimensions but is commonly found smaller. In 

 the female the posterior margin is much more rounded than in 

 the male, and the biangulate character quite disappears. The 

 general outline is more flowingly rounded, and the transverse 

 measurements somewhat greater, in the female than in the 

 male shell. 



