IIISSOELLID.'E. 325 



by which they, occasionally, suspend themselves from the 

 stems of floating gulf-weed, among which they take up 

 their abode ; if the thread by any chance becomes di- 

 vided, the animal emits a bubble enveloped in a glutinous 

 secretion, which rises to the surface, drawing out threads 

 as it ascends, and finally becomes attached to the weeds 

 above. 



Species of Litiopa. 



bombyx, Rang. melanostoma, Eyd. and Soul. 



decussata, Goxdd. obesa, C. B. Adams. 



effusa, C. B. Ada?)is. striata, Pfeiff. 



Fam. RISSOELLID^. 



Lingual membrane with five series of teeth (2*r2), cen- 

 tral teeth broad, crenulated ; lateral, two on each side, the 

 inner broad and crenulated, the outer small and hooked. 

 Rostrum divided into two tentacular lobes in front ; eyes 

 sessile on the bases of the tentacles, or on the head, far 

 behind them. Mantle simple in front. 



Operculum annular, regular, with an internal process. 



Shell elevately spiral ; aperture simple, or slightly emar- 

 ginate in front. 



The lingual armature of this family shows an affinity to 

 the Littorinidce, and the bilobate mouth and absence of 

 retractile proboscis indicate them to be vegetable feeders. 



Genus RISSOELLA, Gray. 



Eyes on the head, far behind the tentacles ; tentacles 

 simple ; lobes of rostrum elongate. 



