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, 

 livijig 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 

 t3q)ically 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 grattdis irihabiting other streams (Wilson and Clark, 1912: 

 61-71). 



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 



