HELICINA. 351 



mellar margin rectilinear, the two margins meeting at 

 the base at an angle. 



Geographical Distribution. In general, this ge- 

 nus is found in company with Cyclostoma. In the United 

 States, however, it is found n^Qt only along the Gulf of 

 ^lexico, but in the Soutliwestei'n and Western States, as 

 far as Ohio. It is abundant in the West Indies, in Bra- 

 zil, and in the high islands of the Pacific. But I know 

 of no species from the west coast of America, from Africa, 

 Europe, New Holland, or indeed from any continental 

 part of the Old World. 



Remarks. This genus was originally instituted by 

 Lamai'ck. He regarded the presence of an operculum, 

 and the peculiarities of the animal, as justifying their be- 

 ing set apart ; but, on account of their general form and 

 habits, he still associated them with the Helicidse. Mr. 

 Say, in examining and describing H. orUculata, in 1818, 

 noticed the same peculiarities, and placed it under a new 

 genus, which he called Oligyra, and also noticed its alli- 

 ance to Cyclostoma. His conception of the limits of the 

 genus, was not very exact, as he afterwards described, 

 under the same name, shells which undoubtedly belong 

 with the Helicidse. The details which the observation 

 of numerous species has since afforded, render it quite 

 certain that the genera Cyclostoma, and Heliclna, and 

 some others derived from them, and we may with almost 

 equal confidence add Truncatella, form a natural group 

 separate from the Helicidie, more nearly allied to the 



