14 THE MOLLUSC A OF MUSCATINE COUNTY. 



TJ. arctior, LEA. Very rare. Among the gibbosus (Barnes) 

 found here is occasional!}' a shell much smaller than the 

 largest gibbosus, nacre bluish white, convex on basal edge, and 

 showing rays more or less distinctly. It is, no doubt, a variety 

 of gibbosus, since the color of the nacre, and form of basal 

 margin, are both variable in gibbosus. 



TJ. capax, GREEN. Rare. Mississippi. Length, 10.8; breadth, 

 7.7 ; diain., 6.8 cm. 



U. coccineus, LEA. Cedar, Iowa, and Des Moines. Some speci- 

 mens from Iowa City are conspicuously rayed with green. 

 Rather common in the Iowa and Des Moines. Length, 7 ; 

 breadth, 5.5 ; diam., 4 cm. 



U. cornutus, BARNES. Common. Mississippi. Epidermis in some 

 examples is closely rayed, with interrupted or zigzag green 

 lines, giving the shell a beautiful appearance. This shades 

 into a light 3*eliow, destitute of ra}~s. One row of increasing 

 tubercles about central in eacli valve : the largest shells have 

 four to five in each row. Length, 5.5 ; breadth, 4.6 ; diam., 

 4.3 cm. 



U. crassidens, LAM. Rare. Mississippi. This species seems to 

 be very nearty related to ligamentmus (Lam.) and gibbosus 

 (Barnes) . It is niger of Raf. A more or less purple nacre, and 

 black epidermis. Length, 14; breadth, 8.1 ; diam., 6.4 cm. 



U. donaciformis, LEA. If this and zigzag (Lea) are two good 

 species, we are unable to sa} r whether it is rare or common. 

 Habitat Mississippi, on sandy bottoms. Lea figured and de- 

 scribed this form in 1827, and zigzag in 1829. We can see no 

 difference in our shells, except some are shorter and more 

 tumid than others, and the epidermis varies from green to 

 yellow. Length, 3.4; breadth, 2.2; diam., 1.8 cm. 



U. dorfeuilliaims, LEA. It is with some hesitation that we put 

 this species in our list. We are unable to separate it from 

 among the shells found in the Mississippi near us, which we 

 regard pustalosus. If Lea's figures and descriptions are cor- 

 rect, we have no shell that quite corresponds to them ; but 

 we can place two examples we have from the Cedar, in this 

 county, nowhere else. Shell very thick and heavy, almost 

 without pustules ; umbones and ligament very prominent. 

 Length, 8.3 ; breadth, 7.5 ; diam., 5.6 cm. 



