40 ILLINOIS BIOLOGICAL MONOGRAPHS [138 
tributary nearby which was dry at the time of our examination. Tetralas- 
mus is a species of the small, mud-bottom tributaries and seems not to 
occur in the larger part of the stream with the larger and heavier mussels. 
The largest specimen collected measures 55 mm. in length. The colors of 
the shells are yellow, black, and greenish, the latter in indistinct ray-form. 
12a. Uniomerus tetralasmus sayi (Ward). 
Among the Anodontas collected in Crystal Lake is a specimen of the 
shell known as variety sayi. It is large for the species but seems otherwise 
typical. The dimensions are: length, 123; height, 58; breadth, 38 mm. 
(No. Z 11369). As only tetralasmus is found in the Salt Fork and its 
tributaries it is probable that this form was introduced with the Anodontas 
described on a subsequent page. 
13. Strophitus edentulus (Say). Squaw-Foot. 
This mussel once occurred in nearly all parts of the Big Vermilion River 
but it is now found in any number only below the dam at Homer Park. 
No living specimens were found above a point four miles above Homer 
Park dam or 22 miles below Urbana. It occurs, rarely, living, in Spoon 
River and its absence in a living state for a distance of 12 miles between 
this tributary and the first habitat in which it was found alive in Salt 
Fork is striking and suggestive of the harmful effect of sewage pollution. 
This species reaches its greatest perfection below the dam at Homer Park 
on a gravel bottom. Ldentulus also occurs in the Sangamon River, but 
the individuals from that stream are not as large and are more compressed 
than the specimens from Salt Fork, which are as a rule quite corpulent. 
It is also not as abundant in the Sangamon as in the Big Vermilion. Meas- 
urements are given below of the largest specimens from the Salt Fork and 
the Sangamon. 
Length, 90; height, 54; width, 41 mm. Salt Fork, Z11174. 
E89 57s S 32mm. Sangamon, Z11227 A. 
There is great variation among the shells referred to this species. 
Typical edentulus is rhomboid in form, rather inflated, with prominent, 
inflated umbones; the posterior margin of the shell is usually sharply, 
obliquely truncated, and the ventral margin is straight or even slightly 
convex. The color is usually black without rays. From this type the shell 
varies to an ovate or elliptical outline, a more or less compressed form, with 
a rounded, convex ventra] margin and with an almost total absence of the 
strong posterior ridge so characteristic of the usualform. The variation is, 
as would be expected, toward the variety known as pavonius. The shells 
from Salt Fork are very thick and solid, much more so than in specimens 
from the Sangamon River. The nacre of the majority of specimens is 
yellowish in color. 
