129] FAUNA OF BIG VERMILION RIVER— BAKER 31 



SYSTEMATIC DISCUSSION OF THE MOLLUSCA 



In this chapter the species of mollusks, both Pelecypoda (mussels, 

 clams) and Gastropoda (snails), are discussed in relation to their distribu- 

 tion in the Big Vermilion River, special emphasis being given the Unionidae 

 or river mussels on account of their economic importance. The influence of 

 sewage pollution of the stream on the molli^sk fauna is also referred to. 

 The species collected in the Sangamon River at Mahomet and elsewhere are 

 included for purposes of comparison. 



The classification followed for the Unionidae is that proposed by Simp- 

 son (1900, 1914) and extended by Ortmann (1912, 1918). The sequence of 

 groups is that set forth in Walker's Synopsis recently published (1918). 

 It will be noted that the newer classification necessitates the adoption of 

 several new names, both generic and specific, but these seem, on the 

 whole, justified by the rules of nomenclature and are a natural result of 

 the advancement of knowledge on the subject. 



For the purpose of providing reliable data on the particular character- 

 istics of the mussels and other mollusks in this stream for comparison with 

 similar features of this group of animals in other streams, a feature almost 

 totally lacking in the literature, considerable space is devoted to descrip- 

 tions of the minor variations and pathological conditions of each species 

 in different environments. This has been done, more or less extensively, 

 in several reports on the mussel faunas of three or four of our Illinois, 

 Indiana, and other streams (Wilson and Clark, Danglade). It will be 

 noted that there are certain features characteristic of the species in one 

 stream not shared by the same species in other streams, as, for example, 

 Lampsilis ventricosa which differs markedly in coloration and even in 

 shape in the two river systems herein considered. Similar data on our other 

 rivers would provide a body of facts of considerable importance, 



FAMILY UNIONIDAE 



1. Quadrula (Quadrula) cylindrica (Say). Rabbits-Foot. 



This species was not found in the Salt Fork above a point about a mile 

 west of its junction with the Middle Fork, 44 miles from Urbana. As it is 

 not listed from the neighborhood of Muncie its westward extension in 

 the stream lies somewhere between Muncie and Middle Fork. Even in 

 this part of the river it is rare and the specimens obtained are small, of 

 dark color and resemble the form called strigillatus by Wright. Cylindrica 

 is not a widely distributed species in Illinois, if one may judge by the 

 records at hand. Danglade (1914) did not find it in the Illinois River 



