CYCLOSTOMID A. 289 
The separation of the whorls commenced in Rhiostona, here 
becomes very striking. A sufliciently large number of s)peci- 
mens were obtained to show that this is anormal form. Syn.—(?) 
Orygoceras, Brusina, 1882. Distr.—C. Mariet, Morel. (1xxvii, 
16). Mayotte Isl. Fossil. Miocene; Dalmatia. 
PrEROCYCLOS, Benson. 
Syn.—Steganostoma, Troschel. 
Distr.—26 sp. India, China, East Indies. P. anguliferus, 
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 lamine. 
SPIRACULUM, Pearson. Distinguished by 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. 8S. hispidum, Benson (1xxvii, 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. 
3sp. Polynesia. D. parva, Pease (lxxvii, 21). 
Catopoma, A. Ad. 
Distr.—C. Japonicum, A. Ad. (Ixxvii, 22,25). Japan. 
Shell subdiscoidal, widely umbilicated ; peristome simple, sub- 
angulated above. Operculum elate, conical, concave, corneous, 
spiral. 
ALycaus, Gray. 
Distr.—54sp. 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. 
Hysocystis, Benson. 
Syn.—P ollicaria, Gould (in part). 
Distr.—3 sp. Burmah, Siam, Cambodia. H. gravida, Benson 
(Ixxvii, 24). 
Shell pupiform; peristome continuous, reflected, with a supe- 
rior process. Operculum multispiral, caleareous, with four or 
five whorls externally, and one and a half whorls internally. 
Family Relations Doubtful. 
CHONDRELLA, Pease. 
Distr.—3 sp. Central Polynesia. CO. parva, Pease, Taheiti. 
Shell globosely conical, rather thin, striate, imperforate or 
