54 ILLINOIS 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 Actinonaias 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 
