42 ILLINOIS BIOLOGICAL MONOGRAPHS [140 
14. Anodonta grandis Say. Floater. 
The floater or paper-shell is more or less abundant in Salt Fork and 
other parts of the Big Vermilion drainage. In Spoon River it is common, 
living in the lower part of the stream. From this station to the station 
called bench mark 655, over five miles below, not a living Anodonta could 
be found, and the species does not become abundant until the cement 
bridge east of Sidney is reached, nine miles below Spoon River. This 
distribution is again indicative of the harmful influence of sewage on the 
bottom inhabiting animals. From the cement bridge to the Homer Park 
dam grandis is fairly common. It was very rare below the station at Homer 
Park, at which place it is common. The best habitat observed appears 
to be between the cement and railroad bridges east of Sidney, where the 
water is fairly deep in summer (three-four feet) and where there is a soft 
mud bottom and not much of a current in the stream. The species is 
typically a pond-inhabiting mussel. Gravid individuals were collected on 
September 13, 1918. 
At Mahomet, on the Sangamon River, grandis is abundant and of large 
size, and occurs on a fine sand bottom. The Sangamon specimens are 
on the whole more cylindrical in form than those from the Big Vermilion 
and have a brown or brownish-green epidermis. The Big Vermilion 
specimens are mostly grass-green in color and are more elongate-ovate 
in form, the ventral margin being almost universally rounded while in the 
Sangamon shells this margin is nearly straight. The Sangamon River 
grandis are on the whole more solid than the same species from Salt Fork. 
The nacre of the great majority of the Salt Fork specimens is bluish- 
white, while that of the Sangamon specimens is salmon-colored for the 
most part. A few individuals from both streams have salmon-colored 
patches and small pearl growths indicating that the animals had suffered 
from the attack of distomid worms, possibly the distomid of Osborn, which 
is known to infest this species in other places (Wilson and Clark, 1912). 
These shells, however, were rare and infection from this source seem 
uncommon among the grandis of these streams. No Unionicola (Atax) 
or other water-mites were observed in this species. These parasites are 
common in grandis inhabiting other streams (Wilson and Clark, 1912: 
o1S71): 
An empty shell from the big bend in the Salt Fork showed evidences 
of distomid infection in the form of elongated blisters on the ventral 
margin of the valves, near the pallial line. In the right valve, near the 
anterior adductor muscle scar, there is a large blister, 8 by 12 mm. which 
evidently covered a distomid. The left valve of this specimen had suf- 
fered an injury when the animal was about two-thirds grown, which has 
caused a part of the antero-ventral margin to become folded inward, a part 
of the folded portion having the epidermis well preserved. The animal 
