CYCLOSTOMA. 303 



Our Continental neighbours are richer 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 divided. 



* Shell oval : operculum testaceous. CYCLOSTOMA. 

 ** Shell cylindrical : operculum horny. 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 solid : whorls rapidly increasing in size : 

 spire short : mouth round : umbilicus oblique : operculum 

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



* Round-mouth. 



