HELIX. 91 



First in importance, we have the common and more 

 normal form of HeHx, of which H. alholahris may be 

 considered as the type, to which belong also H. major, 

 tkyroidus, exoleta, elevata, pennsylvaniea, columhiana, 

 labiosa, hucculenta, clausa^ toivnsendiana, ifo. Some 

 of these are found in every region from the Atlantic to 

 the Pacific, except in the extreme south. 



The lingent species, of which M. tridentata is the 

 type, embi-acing that portion of those denominated Heli- 

 eodonta, by F^russac, which are depressed in form and 

 have generally several pointed teeth ; such as S. fallax, 

 palliata, inflecta, loricata, dejecta. In close proximity 

 to these we have a group of which H. hirsuta is the 

 type, embracmg also IT. sjinosa, edgariana, fraterna, 

 maxillata, and germana. Most of these inhabit a wide 

 extent of the country, though none of them are found 

 at the extreme south. 



The polygyroid group, on the contrary, is confined to 

 the extreme south, and with its type, H. fatigiata, we 

 have H. sejitemvolva, auriculata, 2^ustula, vultuosa, 

 texasiana. They are all of a discoidal form, with more 

 than the usual number of whorls, but with considerable 

 variation in the development of the aperture. 



Another southern form embraces ff. mobiliana, sele- 

 niiia, saxicola, and lerlanderiana, which are small, 

 simple, thin, discoidal shells, entirely dJficrent from all 

 the other North American forms. 



A peculiar type is foimd in California, with which S. 

 aspersa would be numbered, or still more like H. aperta, 



