133] 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 difficult. 
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 rubigz- 
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 
( 
