200 



CYCLOSTOMID^. 



The operculated Pulmonifera differ in such important 

 features of their organization from all other pulmonated 

 Mollusks, that their true position in the series of Gastero- 

 poda is a suhject of dispute, not yet satisfactorily settled. 

 In many points of their anatomy they are distinctly related 

 to the snail tribe ; in others they seem to have affinity 

 with Littorina and similar PectinibrancJdata. Their an- 

 cient embodiment in the genus Turho had a significance 

 even stronger than that indicated by the shapes of their 

 shells. The attention they are now receiving from many 

 able malacologists will probably clear up many doubts re- 

 specting their systematic rank. In the meantime, we 

 describe the very few British representatives of the group 

 at the close of the Pulmonifera, a place not unnatural 

 when we consider the relations oi Acme with Garycliium. 



The genera Cyclostoma and Helicina are the principal 

 types of the family. Of the latter we have no British 

 members, nor, indeed, are there any inhabiting the tempe- 

 rate regions of the globe. The old genus Cyclostoma has 

 been lately divided into numerous lesser but important 

 groups, chiefly characterised by differences in the struc- 

 ture and form of their opercula. In the newly-published 

 " Conspectus Cyclostomaceorum,"" by Dr. Louis Pfeiffer, no 

 fewer than twenty-nine of their genera are enumerated, 

 including above four hundred and fifty species. The ge- 

 neric name Cyclostomus is restricted to an assemblage of 



