neritinA. 



is more or less sharp and straight and generally denticu- 

 lated. The coliimellii^ together with the inner part of the 

 spire, and even a part of the lip is absorbed by the animal 

 in proportion as it increases in size, whence it appears to 

 have no columella. The inside of the outer lip is neither 

 toothed nor crenulated, but just within the lower part of 

 the aperture it has a slightly elongated, transverse pro- 

 minence, which appears to be a fulcrum upon which the 

 operculum articulates. The operculum itself is testaceous, 

 covered with an horny epidermis; of a semicircular form, 

 exactly closing the aperture, and furnished internally at 

 the lower end with a dentiform appendage, which, when 

 the aperture is closed, lies between the above-mentioned 

 prominence in the shell and the lower end of the inner lip. 



The resemblance in the general form of the shell, in 

 the form of the aperture and in the general characters of 

 the opercula, between Nerita and Neritina, is very com- 

 plete ; the principal points of difference between these two 

 genera consist in the inner part of the outer lip of Nerita, 

 being generally occupied by numerous transverse teeth or 

 plaits : the Neritae are, moreover, generally thicker shells, 

 and are frequently ornamented on the exterior with varied 

 grooves, striae, tubercles, &c. Both genera are distin- 

 guished from Natica and from Helix by the straight, 

 sharpish, denticulated edge of their inner lip. We do not 

 conceive it necessary to divide Neritina into three genera 

 as Montfort has done, for his Clithon is a good type of 

 the Genus; and the only peculiarity of his Theodoxus is 

 its small internal chamber, separated from the cavity of 

 the spire by a kind of half septum, and apparently formed 

 after the absorption of the columella. Montfort's Vela- 

 tes is Lamarck's N. perversa. 



Many of the recent Neritince are very pretty little 

 shells, for beneath the epidermis they are frequently orna- 

 mented with variously coloured bands, spots, zigzag lines 

 upon a light coloured ground, and sometimes with light- 

 coloured markings upon a dark ground, and it is re- 

 markable that the fossil species frequently retain these 

 varied markings. Like other fresh water shells the apices 

 of their spires are subject to erosion, and sometimes their 

 spires are almost eaten away, but in the parts that are 

 thus eroded, the animal habitually increases the thickness 

 of its shell to avoid sustaining any injury itself. 



