276 cyclotiNjE. 



Jamaicensis, Cfiem. seminudus, G. B. Adams. 



jugosus, C. B. Adams. semistriatus, Sow. 



laxatus, Sow. stramiueus, Beeve. 



pallescens, C. B. Adams. subrugosus, Sow. 



perpallidus, C. B. Adams. suturalis, Sow. 



Popayanus, Lea. translucidus, Sow. 



prorninulus, Ferus. varians, G. B. Adams. 

 Quitensis, Pfeiff. 



Sub-gen. cyrtotoma, Morch. 



Shell with the last whorl free, cylindrical ; peristome with 

 the left margin entirely free, deeply emarginate at the umbilicus, 

 dilated and expanded below the emargination. 



Mexicanus, Menke. 



Genus OPISTHOPORUS, Benson. 



Operculum calcareous, circular, thick, multispiral, concave 

 on both sides, composed of an internal horny, and an ex- 

 ternal calcareous layer, with an erect, spiral lamina between 

 them, nucleus central, margin concave. 



Shell depressed, orbicular, widely umbilicated, the suture 

 furnished, near the aperture, with an exserted, pervious tube ; 

 aperture circular; peristome double, the external expanded 

 and produced superiorly into a tectiform beak, the internal 

 sometimes emarginate above or shortly excised. 



Syn. Ccelopoma, Benson. 



Ex. 0. biciliatus, Mousson, pi. 85, fig. 2. Operculum, 

 0. biciliatus, fig. 2, a, 2, h, 2, c. 



The species of this group, which is readily distinguished 

 from Pterocyclos by the operculum being flat, and from 

 Myxostoma by the operculum being complex and calcareous, 



