ilx^LACOZOA. GASTEROPODA. FKCTINIBRANCHIATA. 129 



l)resents characters perfectly distinct from those of either of 

 the species just mentioned. 



Genus 2. Neritina, Lamk. 



Animal elongated, spiral ; head broad, with two long, 

 setaceous tentacula ; eyes at their outer base, on a small 

 tubercle ; mouth with two unequal lips ; foot large, 

 depressed ; a horny operculum, marked with lines di- 

 verging from the inner to the outer margin. 



Shell semiglobose or oblong, of moderate thickness, 

 with a persistent glossy epidermis ; the spire very small; 

 the aperture oblique, roundish or elliptical, but reduced 

 to a semiglobular form by the thickening of the columella, 

 which shelves to a thin edge nearly parallel to the axis. 

 Operculum horny slightly calcareous. 



The species, with very few exceptions, are fluviatile. 



1. Neritina fluvidtilis. River Neritine. 



Shell transversely oblongo-elliptical, convex, rather thick ; 

 with the spire very short, and as if lateral, the last turn ex- 

 cessively disproportionate, being extended obliquely to the 

 axis, so as to render the outline of the mouth elliptical, but 

 half filled up by the shelving callosity of the columellar lip, 

 which ends in a thin outer edge ; the margin of the outer lip 

 thin but firm ; the coloiu-ing of the exterior various, banded, 

 tesselated, or spotted. Length about two-and-a-half twelfths, 

 breadth four-twelfths. 



It occurs in slow rivers in England and Ireland, also in tlie 

 Loch of Stennis in Orkney. A perfect shell, but without the 

 animal, was found by me on the 1st of July, 1842, among 

 shell sand on the beach, between the mouth of the Dee and 

 the Don ; and, in September, another was picked up by my 

 son, Paul. The first specimen has the exterior dark olive- 

 green, with numerous oblong, greenish-white spots, disposed 

 longitudinally, the inside bluish-white. 



According to Draparnaud, " the animal is transparent, 

 blackish ; with the foot of a pale colour beneath ; the tenta- 

 cula long, setaceous, very flexible ; the eyes small, black, situ- 

 ated at the outer base of the tentacula on a little tubercle. 

 When the animal walks, it is entirely concealed vmder the 

 shell, and shews only the anterior edge of the head and the 

 tentacula." 



