246 HELICINAD^E. 



fresh water, and marine shells with circular apertures, 

 but afterwards restricted it to the terrestrial species with 

 an operculum. Its limits are not yet well determined, 

 and its natural affinities are by no means settled. Its 

 respiratory system and its mode of life would bring it in 

 connection with the Helicidse ; but its single pair of ten- 

 tacles with eyes at the base, its operculum, and especially 

 its reproductive system, would rather associate it with 

 the Turbinidae. Some are inclined to include in one 

 genus, all the operculated, terrestrial shells, with the 

 exception of Helicina and Pupina, and also the new ge- 

 nus Steganotoma ; while others would subdivide them 

 into several genera. We are disposed to rank with the 

 latter, and to regard the group as a natural family, and 

 to make of it several genera. We think that the great 

 variety of form, texture, and sculpture, authorizes such 

 a subdivision, and the more especially as certain combin- 

 ations of these characters are limited, or nearly so, to 

 certain geographical areas. M. Troschel has based a 

 subdivision upon the structure of the operculum ; and his 

 division has been still further extended by Dr. Pfeiffer. 

 Now, although the organization of this appendage, of 

 itself, may not be sufficient to justify a new generic dis- 

 tribution, yet it must be evident that its form, texture, 

 and the conformation of its spiral, must depend on a 

 modification of the animal ; and it is not a little remark- 

 able that the forms of the operculum are found to vary 

 in each of the local and structural groups above men- 

 tioned. As general examples, we may say that the 



