42 MUSSEL FAUNA OF THE KANKAKEE BASIN. 



investigated. It occurs occasionally in lakes, but is essentially a 

 river shell. A few were found in Tippecanoe Lake. 



In the upper parts of the Yellow River, from Plymouth down to 

 Ober^ and in places in the upper Kankakee this species reaches gi'eat 

 perfection and is represented by a well-defined type not easily con- 

 fused with any other species— a large flat shell of pretty uniform 

 thickness and veiy good luster. The majority of the shells of the 

 upper part of the river are of a delicate pink color, very pleasing to 

 the ordinary eye, but not to the maker of buttons. Farther down 

 stream the majority are white-nacred and serviceable in the com- 

 mercial sense. We were unable to obtain any gravid specimens of 

 this flat, highly characteristic form. Q. cocdnea was fairly common 

 in the clanmier s pile at Momence. 



In some of the lower parts of the Kankakee system, and espe- 

 cially in the Iroquois River, what appears to be this species loses its 

 distinctive character and is represented by a much more inflated 

 form, closely approaching Quadrula solida or some other member 

 of the perplexing oUiqua-tngana-plena-soUda group. A gravid 

 specimen of this inflated form was obtained in the Iroquois River. 

 Only the outer gills contained the glochidia. These filled the entire 

 gill, which was padlike and white in color. According to Simpson's 

 classification this would place it in the genus Pleurobema.^ but more 

 examples are desired and the whole subject requires investigation. 

 Dr. A. E. Ortmann is of the opinion that Q. cocdnea is identical with 

 Q. ohliqua, which usually bears its glochidia in only two gills and 

 which he considers a PleuroheTna. All the examples of Q. cocdnea 

 we have ever seen, however, even the inflated ones, have lacked the 

 peculiar sulcus that is to be found in Q. ohliqua. 



4. Quadrula trigoim {Lea). Pig-toe. —This species is one of the 

 staple button shells. Its size is usually rather small, and a furrow 

 on the valve makes it difficult to cut to advantage, but its excellent 

 luster and whiteness compensate to some extent for its deficiencies 

 and it is excellent for small buttons. Its occurrence in the Kankakee 

 area is exceptional; one was found in the Iroquois at L'Erable and 

 one in that stream 3 miles above its mouth. Another was found in 

 the Kankakee at Wilmington. These are not exactly typical shells. 

 It is somewhat surprising that it is not more common, as it could 

 easily come up from the Illinois River. 



In the upper Mississippi, where the siDecies is abundant, it is very 

 constant in form, and well marked from any other species, but out- 

 side of this range it is quite variable in form. It is very frequently 

 infested by a distomid which forms cysts in the mantle, and it occa- 

 sionally produces beautiful pearls. 



5. Quadrula ruhiginosa {Lea). — This shell is quite similar to 

 Q. cocdnea, from which it is somewhat difficult to separate it. It 



