CYCLOSTOMID^. 289 



The separation of the whorls commenced in Rhiostona, here 

 becomes very stril^ing. A sufficiently large number of speci- 

 mens were obtained to show that this is a normal form. Syn. — (?) 

 Or3'goceras, Brusina, 1882. Dist?^ — C. Mariei, Morel. (Ixxvii, 

 16). Mayotte Isl. Fossil. Miocene; Dalmatia. 



Pterocyclos, Benson. 



Syn. — Steganostoma, Troschel. 



Distr — 26 sp. India, China, East Indies. P. angidiferus, 

 Souleyet (Ixxvii, 17, 18). 



Shell subdiscoidal, widely umbilicated ; peristome expanded, 

 produced into a little wing at the suture. Operculum thick, com- 

 posed of a number of calcareous spiral laminse. 



spiRACULUM, Pearson. Distinguished hj the possession of a 

 retroverted sutural tube open at both ends, and by a modifica- 

 tion of the form of the mantle corresponding to the same. 5 sp. 

 India. S. hispidum, Benson (Ixxvii, 19, 20). 



DIADEMA, Pease. (Garrettia, Pease.) Shell globosely turbi- 

 nate, umbilicated ; peristome continuous, simple, subcircular, 

 free or scarcely adnate. Operculum subcartilaginous, elevately 

 spirally lamellar, concave within, widely reflected at the base. 

 3 sp. Polynesia. D. parva, Pease (Ixxvii, 21). 



COELOPOMA, A. Ad. 



Distr. — G. Japonicum, A. Ad. (Ixxvii, 22, 25). Japan. 



Shell subdiscoidal, widely umbilicated ; peristome simple, sub- 

 angulated above. Operculum elate, conical, concave, corneous, 

 spiral. 



Alyc^us, Gray. 



Distr. — 54 sp, India, East Indies, A. gibbus, Fer. (Ixxvii, 23). 



Shell conical or depressed; whorls rounded, with profound 

 sutures ; last whorl ventricose, strangulated, and tortuous near 

 the rounded aperture ; peristome double, the exterior edge 

 reflected. Operculum corneous, multispiral. 



Hybocystis, Benson. 



Syn. — Pollicaria, Gould (in part). 



Distr. — 3 sp. Burmah, Siam, Cambodia, //, gravida, Benson 

 (Ixxvii, 24). 



Shell pupiform ; peristome continuous, reflected, with a supe- 

 rior process. Operculum multispiral, calcareous, with four or 

 five whorls externally, and one and a half whorls internally. 



Family Relations Doubtful. 

 Chondrella, Pease. 

 Distr. — 3 sp. Central Polynesia. C. pjarva, Pease, Taheiti. 

 Shell globogely conical, rather thin, striate, imperforate or 



