Genus III. RISSO'A* Freminville. PL I. f. 1. 



Body rather slender : mantle furnished at the upper corner 

 (and in some species also at the lower corner) of the mouth of 

 the shell with a minute tentacular process: head depressed 

 above and extended in front, where it forms a long and stout 

 snout-like projection, which is divided at the extremity into two 

 lobes, that serve as lips ; it is armed with a pair of jaws and 

 a very short spinous tongue : tentacles wholly or partially setose 

 or hairy ; tips blunt : eyes on small prominences or tubercles, 

 one at the outer base of each tentacle : gills composed of from 

 a dozen to twenty separate strands : foot lanceolate, narrow, 

 double-edged, broader and more or less truncated in front, some- 

 what contracted in the middle, and pointed behind ; sole 

 grooved down the middle for about half its length towards the 

 tail, whence it emits a glutinous thread by which the animal 

 suspends itself to foreign bodies or to the surface of the water : 

 opercular lobe large, divided into two wing-like expansions : 

 beneath it at its hinder extremity issues a short tentacular 

 appendage, which is in some species double or triple. 



Shell oblong or oval, seldom umbilicate : epidermis very 

 slight : spire usually elongated : mouth oval or trumpet-shaped, 

 angulated above and slightly expanded below ; its lips or mar- 

 gins are continuous. 



The Rissoce are minute, but elegantly shaped : — 



" inest sua gratia parvis." 



They are spread over all the globe — although the tropi- 

 cal seas have not been so well searched as those of the 

 northern hemisphere for such small shells. Of the 25 

 species known in the British Isles, 15 inhabit the littoral 

 and laminarian zones, and 10 the coralline and deep-sea 

 zones. Woodward says that there are altogether 70 

 recent and 100 fossil species. 



i 



* Dedicated to M. Risso, the well-known naturalist of Nice. 

 VOL. IV. B 



75035 



