288 CYCLOSTOMID^. 



lirated. Operculum truncate, conoid, concentric, multispiral ; 

 internally membranous, externally shelly ; external margins of 

 the whorls raised in the form of shelly plates, incurved ; some- 

 times sculptured. 



Animal white, with a short oval foot, undivided beneath ; 

 tentacles small, black, with eyes at the base. 



Cyclotus, Guilding. 



Syn. — Poteria, Gray. 



Distr. — 120 sp. Tropical, world-wide. G. planorbulus, Lam. 

 (Ixxvii, 10). 



Shell subdiscoidal, very widely umbilicated ; aperture circular, 

 entire ; peristome straight or sometimes reflected. Operculum 

 orbicular, calcareous, a little concave exteriorly. 



APEROSTOMA, Troschel. Peristome forming a posteriorly pro- 

 jecting angle, not reflected. Operculum with ridged whorls. 

 G. asperulus, Sowb. 



CYRTOTOMA, Morch (Adams). Last whorl free," cylindrical ; 

 aperture rounded, the left margin dilated, angulated and pro- 

 duced posteriorly. C. Mexicanus, Menke (Ixxvii, 12). 



AMPHiCYCLOTUS, Crosse and Fischer. Operculum horny, mul- 

 tispiral, with a central, wart-like projection inside. Type, G. 

 Boucardi, Salle. Mexico. 



Opisthophorus, Benson. 



Distr. — 16 sp. Siam, Java, Borneo, Sumatra. 0. biciliatvs, 

 Mouss. (Ixxvii, 13). 



Shell depressed, widely umbilicated; suture with a small open 

 tube ; peristome double. Operculum calcareous, rather thick, 

 double, with concamerations between the two disks, which are 

 both concave, the interior one with a corneous epidermis. 



MicHOPOMA, Blanford. Shell turbinate ; epidermis fuscous, 

 thick, hirsute; aperture corrugate within. Operculum multi- 

 spiral, calcareous without, corneous within. 2 sp. Ladia. 0. 

 hirsutuH., Beddome (Ixxvii, 14). 



Rhiostoma, Benson. 



Etym. — Bhion, a promontory. 



Distr. — 6 sp. Siam, Cochin China. 0. Hauyhtoni, Benson 

 (Ixxvii, 15). 



Shell subdiscoidal, broadly umbilicated ; last whorl separate, 

 laterally descending ; aperture free, with an incision at the top, 

 and a subtubular prominence crowning the slit. Operculum 

 multispiral. 



CYCLOSURUS, Morelet, 1881. Shell with three embryonal spiral 

 whorls, then disjoined and becoming tubular ; aperture circular, 

 peristome continuous, simple. Operculum corneous, multispiral. 



