THE MOLLUSCA OF MUSCAT INE COUNTY. 11 



unless fuscus is extremely variable. Shell almost transparent ; 

 apex obtuse. Length, 7 ; breadth, 4.2; height, 2 mm. We 

 have a long, dark form, which answers closely to the descrip- 

 tion of parallelus (Haldeman). Length, 8.6 ; breadth, 4.8; 

 height, 2.5 mm. Many of our dark forms are probably dia- 

 phanus, covered more or less with some foreign substance. 

 So far, they have been found in stagnant water only, and I 

 believe entirely in Muscatine Slough and Keokuk Lake. 



VALVATIDjE. 



VALVATA. 



V. tricarinata, SAY. Abundant in nearly all of our ponds. Two 

 forms, easily distinguished, abound here. We have not yet 

 found both forms in the same pond. But for this we should 

 be disposed to regard one as a prolific, and the other a sterile 

 form. One form is flat, with apex of spire a little below 

 a level with the last whirl ; umbilicus large and shallow : 

 greatest diam., 5.6 mm. The other has an elevated spire, a 

 small and deep umbilicus: greatest diam., 4.6 mm. The 

 latter is a much smaller shell, as is plainly shown in not 

 less than a hundred specimens of each form in our collection. 



VIVIPARID^l. 



VIVIPARA. 



V. intertexta, SAY. Abundant in most ponds, and especially in 

 Muscatine Slough and Keokuk Lake. Some shells show light 

 bands parallel with the sutures. Length, 28 mm. 



V. contectoides, W. G. BINNEY. We have one "dead" shell of 

 this species found on the bank of the Cedar River, at Moscow, 

 in Muscatine Count}-. The shell is in good condition, and 

 plainly indicates the existence of the species near where it 

 was found. Length, 27.5 mm. 



MELANTHO. 



M. subsolida, ANTHONY. Very common. We have the form de- 

 scribed by Anthony. A specimen in my collection has 7^ 



