135] FAUNA OF BIG VERMILION RIVER— BAKER 37 



8. Pleurobema clava (Lamarck). Club-Shell. 



This species is rare in most parts of the Big Vermilion examined, and 

 was not found in the Sangamon. It occurs sparingly at Homer Park, 

 abundantly in the Salt Fork near Muncie, and sparingly in the Middle 

 Fork. At Muncie the largest specimen measured 90 mm. in length. The 

 specimens from the Big Vermilion are beautifully marked with broad green 

 rays on young individuals and on the umbonal half of older specimens. 

 Large individuals are almost rayless. 



This species has been previously known only from the Wabash River in 

 Illinois (Baker, 1906:77) and the present records, although in the same 

 drainage basin, extend the range of its distribution. 



9. Pleurobema coccineum (Conrad). Thin Niggerhead. 



The shell known as coccineum attains large size in certain parts of the 

 Salt Fork. It is common, however, only at one place, below the dam at 

 Homer Park. No shells of this species were observed above the station 

 called bench mark 655, which is about 15 miles below Urbana. The shells 

 from habitats above the dam at Homer Park exhibit evidence of an un- 

 favorable environment, the shells having heavy lines of growth which on 

 some specimens are raised to form ridges. These are especially marked at 

 the rest periods (seasonal). The individuals from the upper part of the 

 stream are also more or less pathologic, 50 per cent of the shells being 

 abnormal in form or with pearly growths on the inside of the valves. 

 Coccineum is found on both a mud and a sand-gravel bottom. 



There is considerable variation in the outline and general shape of the 

 shells from Salt Fork. The outline varies from quadrate to roundly ovate 

 and the ventral margin from nearly straight to strongly convex. Young 

 and half-grown shells seem more uniform than large adult shells. The 

 quadrate individuals may easily be confounded with Fusconaia ruhiginosa. 

 In the last species, however, the shell is more inflated {coccineum is com- 

 pressed), the umbones are directed upward and not backward, as in 

 coccineum, and the posterior portion of the shell has a depressed area and 

 a strong ridge which are absent in coccineum. The nacre of coccineum is 

 pink of various shades, only two specimens from the Salt Fork having 

 white nacre. In this respect the coccineum of the Big Vermilion drainage 

 differ almost constantly from rubiginosa which has white nacre. The 

 epidermis in specimens from the upper part of the stream (Homer Park and 

 above) is usually very dark brown with faint evidences of rays. In some 

 specimens, especially from the station three and a half miles above Homer 

 Park dam, the whole shell, inside and outside, is of a delicate pink shade. 

 Individuals from the Middle Fork and the Big Vermilion, where the species 

 is rare, are lighter in color. The two largest specimens collected from the 

 Big Vermilion drainage measure as follows: 



Length, 96; height, 75 mm. Quadrate form above Homer dam, Z11114 A. 

 " 96 " 80 mm. Rounded form, Homer Park, Zl 1164 A. 



