326 rissoelliDtE. 



Operculum half-ovate, annular, nucleus near the inner 

 straight edge, with a central internal process. 



Shell thin, spiral, conical or subglobose, transparent ; 

 aperture ovate, rounded and simple in front, peristome 

 thin, entire. 



Syn. Jeffrey sia, Alder. 



Ex. R. diaphana. Alder, pi. 34, fig. 6. Operculum, 

 R. diaphana, fig. 6, a, 6, h, 6, c. Shell, R. glabra, fig. 6, d. 



These curious little animals are found adhering to 

 floating sea-weeds, in pools between tide-marks ; their 

 eyes are situated so far behind on the head, that the trans- 

 parency of the shell seems to be essential to the vision of 

 the animal. 



Species of Rissoella. 



conica, C. B. Adams. globularis, Jeffr. 



diaphana, Alder. opalina, Jeffr. 



glabra, Alder. 



Genus HYALA, H. and A. Adams. 



Tentacles with fine setae at the summit; eyes sessile on 

 the centre of their bases. 



Operculum thin, horny, simple, subspiral. 



Shell thin, hyaline, spiral, subconical ; aperture ovate, 

 emarginate anteriorly ; outer lip thin, simple, sti'aight. 



Ex. H. vitrea, Montagu, pi. 34, fig. 7. Shell, H. 

 vitrea, fig. 7, a. 



The foot is simple behind, and there is no filament on 

 the margin of the operculigerous lobe. Were it not for 

 the persistent rostrum, the position of the eyes and general 



