THE MOLLUSC A OF MUSCATINE COUNTY. 23 



ponds and sloughs generally. We have never seen a figure or 

 description of grandis; neither have we received specimens of 

 this species from other localities. We have sent what we 

 call grandis to conchologists all over the country, including 

 England, and we have not had its specific identity doubted. 

 What seems a puzzle to us is the separation of grandis from 

 gigantea (Lea) . The figure and description of gigantea an- 

 swer to what we call grandis. Length, 19.7 ; breadth, 12.5 ; 

 diam., 9 cm. 



A, imbecilis, SAY. Abundant. In all of our ponds and sloughs. 

 Epidermis not so beautiful as those we have received from 

 Ohio. Length, 7.1 ; breadth, 3.4; diam., 2.5 cm. 



A. plana, LEA. Rare. Isett stone-quarry in this city; creek at 

 Cole's Ferry and at Des Moines. Judging from specimens 

 received in our exchanges, plana and decora are badly mixed. 

 We cannot place our shells elsewhere, unless plana and decora 

 are referred to grandis. There can be no doubt that the envi- 

 ronments of the mollusk influence the characters of the shell. 

 A fine specimen from the old Isett quarry measures as follows : 

 length, 16; breadth, 9 ; diam., 6.6 cm. 



A. suborbiculata, SAY. Common, or even abundant, in Keokuk 

 Lake, and lakes along Cedar, especially the lake on the west 

 bank of Cedar, a half-mile below Moscow. Substance of 

 shell thin ; epidermis highly polished, and light color, except 

 in very old individuals it is dark. A fine but rather fragile 

 shell. Length, 17.3 ; breadth, 13.3 ; diam., 6 cm. 



CORBIOULAD^J. 



SPH/ERIUM. 

 S. solidulum, PRIME. Common in streams. Length, 13 ; breadth, 



9 ; diam., 7 mm. 



S, stamineum, CON. Common. Length, 16 ; breadth, 13 ; diam., 



10 mm. 



S. partumeium, SAY. Common in ponds. Length, 15; breadth, 



13 ; diam., 8 mm. 

 S. transversum, SAY. Common in ponds. Length, 15 ; breadth, 



10.5 ; diam., 7 mm. 



PISIDIUM. 



P. compressum, PRIME. Not very common. 



