[75] 



** Unicolored^ with a i^arietal tooth. 

 2. UlOStoma Sayi, Binney. 



Plate 7, figure 4. 



Orbicularly depressed, whorls 5 — 6, thin, regularly obliquely 

 striate : aperture suborbicular, lip white, the margin narrowly 

 reflected and tuberculately dentate on the base, the parietal 

 wall toothed ; umbilicus moderate and deep. Pale horn color. 



Diam. 22, height 15 mill. Cal.V^4.ir.;^^^Y.' 



Maine to Illinois, and southwards to Pennsylvania, inhabiting 

 mountains and elevated districts. 



ME SOD ON, Rafinesque. 



This group embraces most of the larger species of Helices in- 

 habiting North America east of the Rocky Mountains. In 

 Cuba it is replaced by Pachystoma, a genus very closely allied 

 in the form, size and coloration of the shell ; (but in Jamaica 

 the larger species of shells belong to Pleurodonta, quite a dif- 

 ferent type in every respect). A like alliance brings very 

 closely together the species of Ulostoma just described, with the 

 numerous European genus Campyloea^ so that we have in the 

 subfamily Mesodontince first an undoubted American repre- 

 sentative of VaUonia, then a magnified repetition of the same 

 form, with modifications in Ulostoma which is very close to 

 European species. This type of shell in this country appears 

 to have become further modified into the genus Mesodon, in 

 Avhich form it has flourished exceedingly. Upon tracing 3Iesodon 

 southwards, we find the species becoming larger, heavier and 

 more coarsely striate, and thesj; changes culminate in Pachys- 

 toma. The curious relation of the Pachystomge Avith extinct 

 and expiring species of land shells of Madeira is another 

 curious fact in conchological geography. I shall have occasion, 

 more than once before leaving the genera composing the sub- 

 family of which Mesodo7i is the type, to point out among the 

 terrestrial shells of Europe stray individuals of undoubtedly 

 American forms. 



