NERITINA. 23 



In Great Britain this family is represented by a 

 single genus, which contains only one species : — 



NERITI'NA,'' LAMARCK. 



Head provided with a strong proboscis or snout ; tentacles 

 long ; eyes situated on short, slender foot-stalks ; foot broad ; 

 central lingual tooth minute ; first lateral tooth large, sub- 

 triangular, the two next minute ; imcini about sixty, the first 

 large, the rest slender, hooked and denticulated. 



Shell obliquely ovate ; opercuhun calcareous, solid. 



NeRITTNA FLUVIA'TILIS,! LlNNl^. Pl. IV. 



Body yellowish-grey, spotted with black ; head and snout 

 black ; tentacles rather transparent, of a light slaty-grey colour, 

 with transverse streaks of black on the sides ; slightly thickened 

 and widely diverging at their base, slender, and tapering to a 

 fine point ; eyes very large, black ; foot broadly dilated and 

 rounded in front, sole whitish, tail covered by the operculum. 



Shell obliquely ovate, solid, glossy, of a brownish or yellowish 

 colour, chequered with white, brown, purple, or pink, marked 

 with a few strongly-defined lines of growth, and more numerous 

 and finer intermediate striae ; epidermis thin ; whorls three? 

 convex, rapidly and abruptly increasing, body whorl occupying 

 more than two-thirds of the shell ; spire very short ; sutiire 

 rather deep ; mouth transversely semi-lunar ; outer lip sharp ; 

 inner lip flat, glossy, exceedingly broad, continuous with the 

 outer lip ; operculum semi-lunar, glossy, bright yellow or orange- 

 colour, with a few spiral grooves and numerous finer and flexuous 

 striae in the Hne of growth ; outer edge thin, membranous, 

 margined with orange or yellow of a deeper colour, or sometimes 

 with red ; inner edge thick ; spire with from one and a half to 

 two whorls ; the under side is fitted with a small, grooved, sickle- 

 shaped projection, which serves as a lock or fastening by which 

 the operculum is kept in position. 



* Diminutive of Nerita, a genus of marine molluscs, 

 t Inhabiting rivers. 



