ANCYLUS. 265 



Sub-gen. planorbula, Haldernan. 



Shell with the aperture furnished with dentiform plicse, not 

 forming open partitions. 



albicans, Pfeiff. dentata, Dkr. 



Alexandrina, Ehrenb. dentiens, Morel. 



armata, Beck. obstructa, Morel. 

 armigera, Sat/. 



Sub-fam. ANCYLIN^. 

 Shell non-spiral, conical, limpet-like. 



Genus ANCYLUS, Geoffroy. 



Teeth, central 1, lateral 36. Tentacles subulate. Mantle 

 included; pulmonary orifice protected by a branchial ap- 

 pendage. Foot large. 



Shell sinistral, thin, patelliform, depressed, non-spiral, apex 

 directed to the right ; aperture very wide ; peritreme con- 

 tinuous, simple, entire. 



Syn. Angulus, Miihlf. Ansulus, Herrmann. Aney- 

 lastrum, Bourguignat. 



Ex. A. radiatus, Guilding, pi. 84, fig. 5. Shell, A. 

 simplex, Buclioz, fig. 5, a. 



The species of this genus are found in running streams, 

 crawling on the stones and feeding on Ulva and other 

 vegetable productions ; they are met with in South America, 

 Southern and Central Europe, the West Indies, and the 

 British Islands. 



