320 RISSOIID^. 



anteriorly effuse or faintly channeled. Operculum corneous, thick, 

 semilunar, paucispiral, with a claviform process on the internal face. 

 (PL 54, figs. 93, 94.) 



The form of the operculum, and effuse character of the base of 

 the aperture are distinguishing features of this group. The species 

 inhabit warm and temperate seas, are world-wide in distribution, 

 and over 100 good species have been characterized. There are 

 some secondary and tertiary s^^ecies. 

 Section Rissoina (typical). 



Longitudinally costate, with or without spiral strise, aperture 

 produced below, without funicular rib on the base. R. Inca, Orb. 

 Section Rissolina, Gould. 1861. 



With funicular rib below. R. elegantissima. Orb. 

 Section Microstelma, A. Ads. 1863. 



Turreted, ovate, rimate, spire conic, whorls longitudinally plicate ; 

 aperture oblong, produced anteriorly, subcanaliculate, inner lip 

 thickened, outer lip simple. R. d^dala, A. Ad. Japan. 



Section Schwartziella, Nevill. 1884. 



Aperture without basal emargination. R. bryekea, Mtg. 

 Section Phosinella, Morch. 1876. 



Shell reticulated by subequal sculpture; aperture profoundly 

 sinuated below. Operculum denticulate posteriorily. 



R. Sagraiana, d'drb. 

 Section Zebinella, Morch. 1876. 



Shell very finely longitudinally costellate and spirally striate ; 

 aperture widened and emarginate below. R. reticulata, Sowb. 



Section Morchiella, Nevill. 1884. 



Upper part of shell longitudinally ribbed, lower part smooth, or 

 spirally striate. R. gigantea, Desh. 

 Section Zebina, H. & A. Adams. 1854. 



Shell white, solid, opaque, polished, smooth or partly striate; 

 outer lip rather thick, often with one or more anterior internal tu- 

 bercles. R. Browniana, d'Orb. 

 Section Pyramidelloides, Nevill. 1884. 



Narrowly turreted, with spiral ribs, tuberculated below the suture. 



R. IN80LITA, Desh. 

 Section Stossichia, Brusina. 1879. 



Embryonal shell sinistral ; whorls cancellated ; aperture canali- 

 culated in front, lip varicose, dentate within, axis sub^jcrforate. 



