40 

 GENUS PISIDIUM, Pf^k. 



PI$«1I>I1 .n AItl>ITIJM, Prime. 



Shell small, rounded— oval, beak i-aised sliglitly, 

 surface smooth, strife line. Color chestnut ; beaks 

 near the posterior side. Length, three-twentieths of 

 an inch ; breadth, one-tenth of an inch. 



This universally distributed species is abundant, but 

 from its small size seldom noticed. Found in mud in 

 our rivers. 



PINIUlU.n €0.nPRKSSii;!fI, PiuME. 



Shell solid, trigonal, very oblique ; drawn up near 

 the beaks, which are placed posteriorly ; strife dis- 

 tinct ; epidermis chestnut. Size about the same as P. 

 abdltum. Abundant in swales and Illinois river. 



This species varies in shape, but its obliquity is 

 constant. Its peculiarity is the apex of the beaks, 

 which assume the appearance of wings placed on the 

 summit of the umbones. The animal is active. Several 

 other species of this family are reported from Illinois, 

 but I am unable to report any more from this county. 

 Spli. striatinum, Lam., if a distinct species should be 

 included, as we have the shell. The SpJiaeriums are 

 sometimes mistaken by local observers for young 

 Uniones. 



FAMILY UNIONID^,— Naiades. 



SUBCiK.irUS Ui^IO, Retzius. 



Of this great family, fifteen hundred species have 

 been described, and about eleven hundred are admit- 

 ted. We have many species in our rivers, some of 



