AXCHISTOMA. 207 



Pensylvanieum, Green. Zaleta, Say. 



thyroides. Say. 



Sub-gen. poltgyra, Say (Vortex, Beck. Gonostonia, Held., not 

 Bafin. Caracollina, Ehrenb. Euphcmia, Beck. Trigono- 

 stoma, Fitz., not Blainv.). 



Shell umbilicated, arctispiral, depressly orbiculate or dis- 

 coidal ; aperture irregular, often dentate ; peristome labiate, 

 somewhat thickened, reflexed and generally sinuated, often 

 externally scrobiculate. 



affictum, Ferns. infrendens, Gould. 



angigyra, Ziegl. lens, Ferus. 



barbula, Charp. lenticulum, Ferus, 



lulbinum, Desk. Lusitanicum, Pfeiff. 



canaliferum, Anton. obvolutum, Midi. 



Corcyrense, Partsch. plectostoma, Bens. 



diodonta, Middf. profundum, Say. 



discobolus, Shuttl. Rangianum, Ferus. 



fastigiatum, Say. septemvolva, Say. 



gyria, Both. triarium, Friv. 

 holosericum, Stud. 



Sub-gen. drepanostoma, Porro. 



Shell perspectively umbilicated, orbiculate, strongly involute, 

 the last whorl involving the others, thus rendering them con- 

 spicuous in the umbilicus, so that the shell above appears as if 

 umbilicated, 



nautiliforme, Porro. 



Sub-gen. polygyratia, Gray (Ophiogyra, Albers). 



Shell discoidal, flat above, concave at the base ; whorls very 

 widely spirally enrolled; peristome margined, shortly reflexed. 



Charybdis, Ferus. polygyratum, Born. 



