160 ILLINOIS ACADEMY OF SCIENCE 
outside of the shell is found to be covered with the bryozoon 
Plumatella polymorpha. A hydrachnid mite, Diplodontus sp. 
frequently infests the mantal cavity. Dead valves are very 
often found with Ancylus attached to them. 
15. <A. grandis Say var. gigantea Lea—Mr. Wm. A. Marsh 
reports it from the “Big Vermilion River.” No specimens were 
collected which could be referred to this or any of the other 
varieties. 
16. A.implicata (Say)—Only at St. Joseph, Champaign Co. 
The only other Illinois record is that of Mr. Marsh from Lake 
Co. The very great variation within the species of Anodonta, 
even among individuals of the same species, makes it very dif- 
ficult to make accurate determination from shell characters 
alone. Slight differences in the habitat (from muddy to 
gravelly or sandy situations) often produce marked varietal 
differences. The St. Joseph specimens were all dead, and the 
identification was made by Dr. Frank C. Baker. 
GENUS ANODONTOIDES Simpson (in Baker), 1898 
17. A. ferussacianus (Lea) var. subcylindraceus (Lea)— 
Found in all three counties (Monticello, St. Joseph, Crystal 
Lake and Muncie), in small streams or near the sources of the 
larger ones; the habitat is much the same as that of Caruncu- 
lina parva with which species it is usually associated. Fairly 
abundant. 
GENUS ARCIDENS Simpson, 1900 
18. A. confragosus (Say)—Found only at Monticello and 
White Heath. It is a typical Illinois River species, abundant 
at Havana. Like Hurynia anodontoides and a few others, it 
appears to slowly ascend the Sangamon River, but the shallow 
water inhibits its establishment in large numbers. 
GENUS LASMIGONA Rafinesque, 1831 
Subgenus Platynaias Walker, 1917 
19. L. compressa Lea—Hillery, Vermilion Co. Usually 
buried several inches in mud in streams that are neither deep 
nor rapid. It is fairly common. 
