RissoiD^. 259 



epidermis, exteriorl}^ thickened lip, horny operculum, and want 

 of long pointed apophysis. 



Iravadia, Blanford, 1861. 



Distr. — S. ornata, Blf. Brackish water, India. 



The shell has the general form of a Rissoa, but the apex is 

 often obtuse, the whorls are spirally ribbed, covered with an 

 epidermis ; aperture ovate, with continuous margins, anteriorly 

 slightly effuse ; outer lip with an external varix. Operculum and 

 animal unknown. 



Hyala, H. and A. Adams. 



Distr. — European. H. vitrea, Forbes and Hanley (Ixxi, 22). 



Shell thin, hyaline, elevated conic ; aperture oval, slightly 

 emarginated anteriorly ; outer lip thin, simple. Operculum 

 thin, corneous, simple, subspiral. The tentacles of the animal 

 have bristle-like summits. 



Tatea, Woods. 

 Bistr. — Tasmania. T. Huonensis, Woods (Ixxii, 30). 

 Shell elongate-pyramidal. Operculum calcareous, with a ver- 

 tical, submarginal claw. Animal with truncate foot, and long 

 tentacles. 



The operculum has the form of that of Rissoella, but differs 

 in being calcareous. 



Family RISSOID^. 



Shell small, spiral, turreted or depressed, often more or less 

 umbilicated ; aperture more or less rounded, never truly chan- 

 neled in front ; peritreme continuous. 



Tentacles elongated, with the eyes at their outer bases. Verge 

 (male organ) exserted, situated on the back at a considerable 

 distance behind the right tentacle. Gills both pallial ; the right 

 or principal one usually rather short and broad, and composed 

 of few laminae, which are much broader than high. Foot oblong, 

 punctate before, rounded or pointed behind. Operculigerous 

 lobe well developed. Operculum horny or partly shelly, spiral 

 or concentric. Lingual teeth, 3"l-3 ; living in fresh, brackish or 

 sea-water, sometimes amphibious. Distribution mundane. 



Stoliczka indicates two principal groups in this family : the 

 first including the marine genera, with thick, solid shells, and, 

 as a rule, with the labrum externally thickened ; the other the 

 brackish and fresh-water or amphibious genera, the shells of 

 which are usually thin, smooth, with an olivaceous epidermis, 

 the labrum not externally thickened. The animals of all the 

 Rissoidse are very similarly formed, but those living in fresh or 

 brackish waters have generally no appendages on the posterior 

 portion of the foot, and the operculigerous lobes are less devel- 

 oped than in the marine forms. The eye-peduncles are generally 



