157] FAUNA OF BIG VERMIUON RIVER— BAKER 59 



shallow water on a more or less rocky bottom a short distance below 

 the dam. 



41. Musculium transversum (Say). 



This fragile finger-nail clam is apparently rare in Salt Fork and its 

 tributaries, occurring only in Salt Fork near Sidney. It has been reported 

 from Crystal Lake by Zetek. The specimens are typical. 



42. Musculium truncatum Linsley. 



This very small clam was found only in the old river bed near the 

 cemetery north of Urbana. Here it is common and typical. 



43. Musculium partumeium (Say). 



Reported by Zetek from Crystal Lake. It has not been found recently. 



44. Pisidium compressum Prime. 



This small clam occurs in abundance only at the iron bridge north of 

 Sidney. A few specimens were found below the natural dam in Salt Fork. 

 The individuals are quite typical. 



45. Pisidium kirklandi Sterki. 



A single specimen of this species was foumd at the iron bridge north 

 of Sidney. It is apparently very rare. 



46. Pisidium splendidulum Sterki. 



A few specimens of this tiny species were found in the Salt Fork near 

 Muncie. The individuals are young and not quite typical. 



The distribution of the Sphaeriidae in Salt Fork is interesting and 

 significant in connection with the sewage pollution of the stream. It 

 will be noted that no Sphaeriidae were found in the Salt Fork between the 

 Urbana ditch and the station called natural dam, 14 miles below Urbana. 

 These mussels are characteristic mud dwellers and their absence from the 

 intervening territory in the stream is striking evidence of the unfavorable 

 conditions on the bottom. They were not found in abundance above 

 bench mark 655. It will also be noted that Sphaerium appears, living, 

 before Pisidium and is also found in abundance higher up the stream. 

 This may mean that Sphaerium is better able to withstand these adverse 

 conditions than Pisidium, which is more of a mud dweller. Pisidium is 

 much less common everywhere in the stream and but one species occurs 

 abundantly at one place (Table IV). 



FAMILY PLEUROCERIDAE 



47. Pleurocera elevatum (Say). 



This long-spired snail is rare or wanting in most parts of the Big 

 Vermilion River. Two white, weathered specimens of this species were 

 found below the natural dam in Salt Fork. They may have been washed 



