133J FAUNA OF BIG VERMILION RIVER—BAKER 35 



(Fig. 27). Still another shell had covered a quantity of mud, which had 

 gotten in between the mantel of the animal and the shell, with a thin layer 

 of pearl, forming a large pad-like blister covering the greater part of the 

 interior of the left valve (Fig. 26). The right valve was normal (Fig. 25). 

 These blister pearls, as well as the more valuable free pearls, are believed 

 to be caused by parasites, perhaps distomids. These injured shells are 

 eagerly sought by the pearl hunters and mussel fishermen in the belief 

 that they may contain pearls of value. 



The undulata from the Sangamon River also exhibit a wide range of 

 variation in the shape of the shell, but not to the degree seen in the material 

 from the Spoon River and Salt Fork of the Big Vermilion River. In the 

 Sangamon, undulata is very abundant on a gravel and sand bottom. 

 In the branches of the Big Vermilion it occurs on both a mud and a sand- 

 gravel bottom. As this species is very successful in resisting adverse 

 conditions it is a valuable mussel for propagation in the streams subject 

 to pollution. It is probably not much affected by a moderate amount of 

 sewage in its environment. The button manufacturers consider it a good 

 shell when the undulations are not too heavy to render the cutting of 

 blanks difi&cult. 



7. Fuscunaia rubiginosa (Lea). Wabash Pig-Toe. 



This mussel is abundant in the lower part of the Salt Fork. It was not 

 found in any abundance above the dam at Homer Park, where it is abun- 

 dant, and the large number of dead, empty shells observed attest the 

 presence of an unfavorable environment. As it is rare in Spoon River, 

 where some other species are abundant, it is probable that this species 

 requires fairly deep water and a large stream bed to attain good size and 

 abundance in individuals. This characteristic of distribution was also 

 noted by Wilson and Clark (1912:43) in the Kankakee River where ruhigi- 

 nosa was found to be more common in the lower part of the river. 



There is considerable variation in the form of the shell; some examples 

 are compressed, others quite inflated. Nearly all are distinctly quadrate, 

 but in some examples the ventral margin is convex; in others it is some- 

 what concave; while in a few it is straight. The individuals from the 

 upper part of the stream, above Homer Park dam, are usually dark brown 

 with a satiny sheen to the epidermis and are almost rayless. Those from 

 the lower part of the river, especially from Middle Fork, are light yellowish 

 brown, quite distinctly rayed. The nacre varies from white to pink or 

 salmon, but is white in the great majority of specimens collected. Young 

 shells 28 mm. in length were common below the Homer Park dam, but were 

 apparently rare above the dam. This may indicate adverse conditions 

 due to sewage pollution and the species may not now be breeding freely, 

 possibly for lack of suitable fish for the glochidia. Young specimens were 



