THE MOLLUSC A OF MUSCATINE COUNTY. 21 



greater abundance of food in Mud Creek." Lea says this 

 species is symplrpiote. We have before us good representa- 

 tives of both forms, a is perfect!}' symplrynote, with a very 

 fine dorsal wing, making the disk somewhat triangular ; b is 

 entirety destitute of a wing, and consequently is not symphy- 

 note, and could not be called triangular ; a would not be 

 transformed into b by loosing its wing ; hinge-teeth are much 

 heavier in b. Measurements of a, length, 14.2 ; breadth, 

 including wing, 12.3 ; diam., 3 cm. Measurements of 6, 

 length, 15.3; breadth, 9.8; diam., 5.2 cm. It will be seen 

 that b is much more tumid than a. The form 6 is common in 

 Mud Creek at Wilton. 



M. confragosa, SAY. Rare. Sloughs and ponds near Missis- 

 sippi. Lateral teeth plainly present in some examples. 

 Length, 11 ; breadth, 8 ; diam., 5.4 cm. 



M. deltoidea, LEA. Mad Creek and Honey Creek, Delaware 

 County, Iowa. In Mad Creek, from two to four miles from 

 its mouth, we have found "dead" shells of this species 

 along with "dead" shells of A. ferussaciana (Lea). It is 

 alive in Honey Creek : it seems to have become extinct here 

 recently. Calceola (Lea) appears to be the same as deltoidea. 

 Length, 2.9 ; breadth, 1.7; diam., 1.1 cm. 



M, marginata, SAY. Very rare. Mississippi, Cedar, Des 

 Moines. We found it in Burdett's Slough. Our shells are 

 very light and small. A specimen from Des Moines measures, 

 length, 6.8; breadth, 4; diam., 2.6 cm. 



M, rugosa, LEA. Mississippi, Cedar, Des Moines. Rare. We 

 found it at Wyoming Hills and in Burdett's Slough. Length, 

 11.1 ; breadth, 6 ; diam., 3.8 cm. 



ANODONTA. 



A. corpulenta, COOPER. Muscatine Slough. Rare. Substance of 

 shell very thin ; hinge margin straight ; ligament hid, or nearly 

 so, by the shell, which is more or less continuous from one 

 valve to the other ; umbones prominent, with three or four 

 coarse folds about the beaks ; epidermis yellowish green ; 

 basal margin very convex. We are in doubt about this spe- 

 cies. Length, 7; breadth, 5.2; diam., 3.4 cm. 

 A. decora, LEA. Muscatine Slough. Rather rare. We have 



