58 ILLINOIS BIOLOGICAL MONOGRAPHS [156 



and Wabash rivers. The small size of the Danville specimens may be 

 due to the fact of their inhabiting the upper part of their distributional 

 range, in a rather small river. Rafinesque's name torulosa has been 

 used by Conrad, Agassiz, Reeve, and Ortmann for Truncilla perplexa 

 and if the original description is clear enough to reasonably fix the shell 

 as Lea's perplexa this form will stand as Truncilla torulosa rangiana 

 (Lea). Neither perplexa or rangiana are known from the Sangamon River. 



FAMILY SPHAERIIDAE 



37. Sphaerium striatinum (Lamarck). Variety. 



This finger-nail clam occurs abundantly only in the ditch north of 

 Urbana. It also occurred infrequently in Spoon River, and near the 

 iron bridge in Salt Fork north of Sidney. Dead specimens were 

 found at bench mark 655 and below the Homer Park dam. 



Sterki remarks of this form "possibly a form of striatinum Lam; 

 apparently the same as a Sphaerium from Iowa and other places which for 

 many years has been left unnamed, except in manuscript. It still seems 

 impossible to say where the limits of striatinum are." The writer fully 

 agrees with Dr. Sterki regarding the variation in this common species. 

 This variety appears quite distinct from the ordinary specimens of the 

 species and it is to be hoped that Dr. Sterki wUl give it a name. It is 

 noteworthy that the form occurs commonly only in the drainage ditch above 

 Urbana and was not found in any numbers below this place. A Sphaerium 

 identified as striatinum by Zetek some years ago, occurring in Crystal Lake, 

 was doubtless this form. 



38. Sphaerium stamineum (Conrad). 



This small mussel is abundant at several places in Salt Fork above 

 Sidney. It occurs only infrequently at two other places. Reported by 

 Zetek from Crystal Lake. Some of the shells are typical but others are 

 similar to straitinum and it is difi6cult to separate some specimens from that 

 species. At the station called bench mark 655 a form of this species 

 occurs that is somewhat like the variety forbesi Baker, described from 

 Mason County, Illinois. 



39. Sphaerium solidulum (Prime). 



An abundant species in the Spoon River and in the Salt Fork above 

 Sidney. Occurs rarely at other places. Reported by Zetek from Crystal 

 Lake. On the whole this species is quite uniform in characters. 



40. Sphaerium species. 



A Sphaerium occurs abundantly in Salt Fork below the Homer Park 

 dam which is thought by Sterki to be possibly a new species. It is related to 

 solidulum but is larger and more robust (mus. no. Z11383). It lives in 



