RISSOIDiE. 327 



character of the shell would indicate PyraiiiidellidcB to be 

 the best position for this genus. 



Fam. RISSOID^. 



Lingual membrane with the inner lateral teeth very 

 broad, the apices incurved, lobed ; outer laterals dissi- 

 milar, all with denticulated apices. Rostrum more or less 

 adnate, below, to the fore part of the foot ; tentacles seta- 

 ceous, with the eyes on bulgings at their outer bases ; 

 neck-lobes none. Foot angulated in front, acuminate 

 behind ; operculigerous lobe with developed lateral expan- 

 sions, and usually furnished with a caudal, tentacular 

 filament. 



Operculum horny, subspiral. 



Shell generally white, spiral, more or less turreted ; 

 aperture usually simple in front. 



Genus RissoiNA, D'Orbigny. 



Operculum semilunar, subspiral ; muscular impression 

 longitudinal, with an elongated process before it. 



Shell turreted, ribbed or cancellated, many-whorled ; 

 spire acuminate ; aperture ovate, effuse anteriorly, slightly 

 channelled in front; outer lip anteriorly dilated, thickened 

 internally. 



Ex. R. Cumingii, Reeve, pi. 35, fig. 1. Operculum, 

 R. cgelata, A.Adams, fig. 1, a, 1, h. 



The species of this genus are very numerous, and are 

 usually regarded as Rissoce ; the mouth of the shell being 

 somewhat canaliculated, and the peculiar operculum, 



