131] FAUNA OF BIG VERMILION RIVER—BAKER 33 
necessary to these animals. The young shells (20mm. in length) are almost 
smooth with a broad dark green ray or stripe extending from the umbones 
to the ventral margin of the valve. No young shells were found in the 
stream above Homer Park dam and the species may not be breeding in 
this part of the stream at the present time. 
The pustulosa from Salt Fork exhibit some variation in the pustulosity 
of the surface, but all are referable to typical pustulosa. A few specimens 
from Homer Park are more quadrate than the average but are otherwise 
typical. The individuals from the Sangamon River, however, show con- 
siderable variation in both form and pustulosity, ranging from nearly 
circular to quadrate and from almost smooth to quite pustulose. On an 
average, however, they are less pustulose than the species occurs in Salt 
Fork. Individuals might be picked out that could be referred to both 
dorfeuillianus Lea and schoolcraftensis Lea, but the range of variation is so 
great that they seem better referred to pustulosa. In the Sangamon River 
pustulosa occurs on a sand and gravel bottom but in the Big Vermilion 
River it is found most abundantly on a mud bottom. The individuals 
from Homer Park and the Sangamon River, especially the later, are of 
good size, 70 to 80mm. in length, and the thickness of the shell combined 
with the clear pearly luster would seem to make them good shells for the 
button trade. No evidences of parasitism were observed in the shells 
examined. 
4. Quadrula (Theliderma) lachrymosa (Lea). Maple-Leaf. 
This handsome shell is very rare in the Salt Fork and was not found 
at any of the localities in the Sangamon River. In the Salt Fork it has 
been found only at Homer Park below the dam, and only occasional speci- 
mens have been collected here. These are quite typical of the species. 
5. Tritogonia tuberculata (Barnes). Buck-Horn; Pistol-Grip. 
The buck-horn first makes its appearance in Salt Fork at Homer Park 
below the dam where it is of large size (female 145, male 115 mm.) and fine 
quality. The shells are densely covered with tear-like pustules which in a 
few individuals cover the entire surface, though usually confined to the 
middle and anterior end behind the posterior elevated ridge. Of the speci- 
mens collected 40 percent are males. Young specimens 46 mm. in length 
were found at Homer Park. Individuals from Homer Park are larger than 
those collected in the Sangamon River, the largest specimens being found 
on a mud bottom, although it also lives ona sand and gravel bottom. Both 
the Salt Fork and Sangamon specimens are of good quality from the button 
makers standpoint. 
Abnormalities and pearly growths due to injuries or parasitism are 
rare in the specimens of this species examined. A few individuals from 
Homer Park had scattered pin-head pearls and a small patch of discolored 
( 
