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 pusttflose. 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 schooler aftensis 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 on a 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 



