70 PLEUROTOMARIIDiE. 



The genus includes several hundred fossil forms, mostly Paleo- 

 zoic. It is one of the oldest Gasteropod genera, commencing in the 

 Cambrian. There have been published several valuable articles 

 upon the recent species, among them Mr. Crosse's Monograph in the 

 Journal de Conchyliologie for 1882, and Dr. Dall's study of the soft 

 parts of P. adansoniana and quoyana in the Blake Report, making 

 known for the first time the anatomical structure of this most inter- 

 esting type. The recent species "are four in number. About a dozen 

 specimens are known, in all. They fall into two sections. 



Section Perotrochus Fischer, 1885. Form conical ; base not 

 umbilicated ; w T horls striate or granulate. Anal fasciole submedian 

 or below the middle ; slit short. (Type P. quoyana.) 



Section Entemnotrochus Fischer, 1885. Shell conoidal, striate, 

 umbilicate ; anal fasciole a little above the middle of the upper sur- 

 face of the last whorl ; slit long, but not much exceeding the half of 

 a whorl. (Type P. adansoniana.') 



Section Perotrochus Fischer. 

 P. quoyana Fischer & Bernardi. PL 56, figs. 4, 5, 6. 



Shell trochiform, obtusely carinated, with the base rounded, flat- 

 tened and concave but not umbilicated. Spire turbinate, terminat- 

 ing in an acuminate apex ; whorls 9, slowly increasing, rounded and 

 swollen toward the suture, divided into two unequal portions by the 

 slit fasciole, granulose. Slit fasciole below the middle, decussated by 

 semicircular and spiral stria?. Slit quite wide, but short ; umbilical 

 depression excavated, nacreous, iridescent, and surrounded by slight 

 concentric grooves. Aperture semioval, nacreous within. Col- 

 umellar margin recurved with a nacreous callosity ; outer lip not 

 reflexed, nearly sharp. Color pale rose, with obscure dashes or 

 flammules of reddish-brown. 



Alt. 40-42, diam. 48-50 mill. ; width of slit, 2% mill. Operculum 

 nearly circular, brown, with 10 whorls, 7* x 7 mill. 



Off Barbados, 73 and 84 fms. ; Off Yucatan, 130 fms. (dead) ; Id. 

 of Marie- Galante, near Guadelupe. 



P. quoyana Fischer & Bernardi, Journ. de Conchyl. v, p. 165, 

 t. 5, f. 1-3, 1856.— Crosse, 1. c. xxx. p. 14, 1882.— Sowerby in 

 Reeve's Conch. Icon., f. 2,1874. — Dall, Report on Blake Gasterop., 

 p. 397, t. 29, f. 1; t. 31, f. 1; t. 37, f. 5. 



The smallest of the recent species. The animal is described by 

 Dall, /. c. The above description is taken from that of Crosse. 



