54 ILUNOIS BIOLOGICAL MONOGRAPHS [152 



Shells from the Sangamon River are equally large. 



The shape of the shell is fairly constant and there is little variation 

 except in the females, which are rounder posteriorly and have a large post- 

 basal swelling for the enlarged marsupia. In color the Big Vermilion shells 

 are all yellowish with dark green rays on the posterior slope. No specimen 

 was seen in this river that was rayed all over. Very old specimens are 

 entirely rayless. At Mahomet, on the Sangamon River, the yellow shell 

 occurs and also another form in which the shell is yellowish-green with 

 bright, grass-green rays, often of considerable width. One specimen is 

 in outline like AcHnonaias ligamentina and the surface is densely covered 

 with dark green rays. Two other specimens have green rays on a pink 

 background, have pink hinge teeth, and the whole interior of the shell is 

 pinkish. These bright colored shells are the form called occidens by Lea. 



These pink shells and the specimens with the numerous green rays are 

 so strikingly different from the ventricosa as found in the Big Vermilion 

 and also from the other shells found in the Sangamon, that the name 

 occidens might be retained for these shells for ecological purposes. The 

 color is not an age stage, for young yellow ventricosa were found associated 

 with these distinctly rayed forms, and the rayed forms were collected 

 at Mahomet and were not found at White Heath in the Sangamon. The 

 river below Mahomet has not been carefully searched, however, and the 

 occidens form may occur in some of this unexplored territory. It is also 

 to be noted that, as far as the material from these two rivers is concerned, 

 the beak sculpture of the occidens type of shell is very much larger and 

 coarser than in the ventricosa type (Z11222). This form falls under the 

 group of individuals called mutations by DeVries and others. 



Ortmann (1918:583) makes ventricosa a variety or race of ovata (Say). 

 As far as the authors' experience goes, this seems unwarranted, the two 

 species being as easily separable as many other closely allied species. 

 Individual specimens from Illinois localities approach ovata in that the 

 posterior ridge is somewhat accentuated, but no specimens have been seen 

 that could not be placed readily in one species or the other. Ovata is 

 reported from the Ohio River in Illinois by Marsh but this species is more 

 southern in its distribution, reaching its maximum development in Ala- 

 bama and Tennessee. Ventricosa is a more northern species attaining its 

 maximum development in the rivers of Illinois and Indiana. 



Ventricosa is subject to the attack of distomid worms as well as to 

 parasitism by mites (Unionicola) and the material of this species from the 

 Big Vermilion have suffered more or less from this cause. Blisters, pin- 

 head pearls and various abnormalities occur in many individuals. Among 

 the shells collected at the station one mile above iron bridge north of Sid- 

 ney there are several individuals of this sort. One has a large blister ex- 

 tending nearly the whole length of the ventral margin and covering the 



