202 BRITISH MOLLUSKS. 



which are placed parallel rounded plates. Operculum pauci- 

 spiral, furnished on the under side with an apophysis which 

 hinges on the columella. 

 Shell ; obliquely ovate, of three very rapidly enlarging whorls, aper- 

 ture transversely semilunar, columella broadly callous. 



This very interesting little mollusk is the single European mem- 

 ber of a group which is very abundant in the warmer latitudes of 

 our hemisphere, both in the rivers and on the seashore. Of the 

 river species, the Neritince, about a hundred and thirty are known, 

 of which at least eighty are natives of the Philippine Islands and 

 the islands of Polynesia. Four have been described from Mada- 

 gascar, ten from Sumatra and Ceylon, four from the Ganges, six 

 from Africa, and five from Australia. The remaining twenty are 

 all within a limi ted area of the New World, enclosing the "West 

 Indies and Central America. The marine species of this family, 

 the NeritcB, which are less numerous, about ninety in number, have 

 much the same distribution, excepting that in the Eastern Hemi- 

 sphere they do not come nearer to us than the latitude of Senegal 

 and the Bed Sea, while on the western coast of Am erica they reach 

 in a southerly direction to Peru. 



The animal of Neritina is comparatively small and obtuse, nearly 

 covered by its rather massively developed shell, and has only a 

 moderately protruded proboscis. The tentacles are slender, con- 

 tractile, and flesuous according to the will of the creature, and con- 

 stantly in motion, and the eyes are raised on short, separately 

 detached stalks. The foot, as may be observed in our vignette, is 

 broadly obtusely dilated in front as far as the extremity of the pro- 

 boscis. As may be gathered from the comparatively solid and 

 overwhelming growth of the shell, Neritina is not a mollusk of 

 active habits. It does not float on the surface, but adheres to stones, 

 or crawls upon a stony or gravelly bottom. The shell is of a peculiarly 

 oblique form, arising from its rapidly enlarging narrowly produced 

 growth of scarcely three volutions. The operculum, which is testa- 

 ceous and paucispiral, peculiarly is characterized by the presence of 

 a hooked apophysis on the underside next the columella, on to which 

 it slides and acts like a hinge. 



Our British species of Neritina is : — 



1. fluviatilis. Shell an oblique oval of three rapidly enlarging 

 whorls, variously reticulately painted, having the columella 

 transverse and broadly callous. 



