174 BRITISH MOLLUSKS. 



Family I. CYCLOSTOMACEA. 



Animal having the head produced into a ringed or ridged proboscis, ten- 

 tacles bristle-like, with the eyes at their outer base, foot bearing on the 

 upper posterior surface a shelly or horny operculum. 



Except in intertropical countries, from whence more than a 

 thousand species of this family have been described, the operculum 

 is an apparatus belonging almost exclusively to the water-dwelling 

 mollusks. We have in Britain only two operculated niollusks re- 

 spiring and dwelling in air, each the type of a distinct genus. 

 They live in damp places or places near the sea, and have a certain 

 affinity with the Auriculacea of simdar habit among the inopercu- 

 lated tribe. The head, which in the Auriculacea is enlarged into a 

 ringed muzzle, is in this family produced into an absolute proboscis, 

 and the tentacles are also bristle-like, with the eyes at their outer 

 base. The genera Cyclostoma and Acme present much the same 

 contrast in their resemblance as Conovulus and CarycMum. One 

 is comparatively of large size, carrying a solid testaceous shell, the 

 other is very small, carrying a transparent glassy shell. 



Our British Cyclostoma is the northernmost member of a group 

 which, like Physa, is abundantly represented in the West Indies, 

 but throughout all Europe has scarcely more than two other 

 species nearly allied, and a few small species (Pomatias) in which 

 the relationship is more removed. Unlike Physa and Lymncea, 

 which so abound in the United States, Cyclostoma does not ap- 

 pear in the New World north of Florida, and then only in a 

 single species, very closely allied, if not identical with, one belong- 

 ing to Cuba. The most striking example of the presence of this 

 genus in the Eastern Hemisphere, is at Madagascar and the 

 neighbouring islands of Bourbon and Bodriguez. Here Cyclosto- 

 ma, with an operculum corresponding in structure with that of our 

 own species, produces a bold globosely turbinated shell, richly co- 

 loured and variously banded and keeled, numerous in species, but 

 all stamped with a well-defined local pecidiarity of character. The 

 operculum of these is, indeed, more like that of our own species than 

 the operculum of the more closely resembbng, in other respects, 

 West Indian species, — a flat rotary constructed calcareous plate, 

 with a central nucleus, like the apical nucleus of the shell, enlarged 

 spirally as the shell enlarges. 



The other British genus of this family, Acme, is an extremely 

 smaU mollusk, with a turriculate glassy shell, scarcely the twelfth 



