CYCLOSTOMA. 303 



Our Continental neighbours are riclier than ourselves 

 in genera as well as species belonging to this family^ 

 Of the typical genus^ Cyclostoma, two well-marked spe- 

 cies are common in France^ although one of them (C. 

 sulcatum) is confined to the South. As to Acme they 

 boast of having four species to our one ; but none of the 

 three which we want are found in the North of France. 

 Of a third genus, Pomatias, no less than six species are 

 French, while we have no representative of the genus. 

 Two of these last species (viz. P. septemspiralis or macu- 

 latus and P. obscurus) have been lately detected in the 

 extreme North of France ; and it is therefore not impos- 

 sible that they may be also met with in the South of 

 England. Their operculum is horny, instead of shelly as 

 in Cyclostoma; and the genus to which they belong 

 appears to bear nearly the same analogy to Cyclostoma 

 as Hydrobia does to Bythinia among the Pectinibranch 

 Mollusca. 



The British genera comprised in this family may be 

 thus di\dded. 



* Shell oval : operculum testaceous. Cyclostoma. 

 ** Shell cyhndrical : operculum homy. Acme. 



The position of the male organ of reproduction is also 

 different in these genera. 



Genus I. CYCLO'STOMA *, Draparnaud. 

 PL VIII. f. 5, 6, 7, 8. 



Body oblong, always containable within the shell : tentacles 

 cylindrical, with slightly swollen tips : foot small and broadish. 



Shell oval, rather soHd : ivJiorls rapidly increasing in size : 

 spire short : mouth round : umhilicus obhque : operculum 

 roimdish, testaceous and solid, with a nearly central spire. 



* Eound-mouth. 



