232 LITTORINID^. 



Genus PURPURINA, d'Oib, 1850. 

 Shell narrowly perforate, turbinate, ventricose, thick, whorls 

 rounded, channeled at the suture, longitudinally ribbed, spirally 

 striate; aperture oval, subchanneled in front, columella arcuated, 

 lip simple. P. Bellona, d'Orb. (PI. 40, hg. 78.) Jurassic. 



Genus MODULUS, Gray, 1840. 



Narrowly umbilicated, top-shaped, solid, grooved or ribbed and 

 tuberculate ; columella grooved, ending below in a sharp tooth. 

 Operculum multispiral. Animal with long, cylindrical tentacles, 

 bearing eyes at about half their length, mantle margin fringed, 

 foot truncate in front, attenuated behind. M. tectum, Gmel. 

 There are about a half dozen species, living in warm seas. Denti- 

 tion, PL 40, fig. 71. 



Genus RISELLA, Gray, 1840. 



Shell trochiform, with flat whorls, angular periphery and con- 

 cave base, imperforate ; aperture rhomboidal, lip simple, usually 

 brown-margined within. Operculum paucispiral. 



Eyes on short peduncles at the base of the tentacles. Dentition, 



PI. 40, fig. 67. R. MELANOSTOMA, Gmel. 



Australasia. 



Subgenus Peasiella, Nevill, 1884. 



Shell umbilicated or perforate. Operculum concentric, multi- 

 spiral. R. TANTILLA, Gould. 



Polynesian, Indian. 



Subgenus Plesiotrochus, Fischer, 1878. 



Imperforate, elongately conic, whorls numerous; aperture sub- 

 rhomboidal, smooth within, prolonged into a short canal below, lip 

 subrostrate in the middle. R. Souverbiana, Fischer. 



Netv Caledonia. 



Subgenus Limnotrochus, E. A. Smith, 1880. 



Umbilicated, conic, spirally ridged, aperture smooth within, the 

 outer lip oblique, basal margin broadly sinuated, columella some- 

 what reflected, unitctl to tlie lip above by a callosity. 2sp. 



Lake Tanganyika, E. Africa (fresh water). 



Scarcely separable from the pi-eceding exce])t by its different 

 habit and locality. R. Thomsoni, E. A. Smith. 



