GASTROPODS. 
395 
FRESHWATER NERITE, 
Neritina (nat. size). 
former there are nearly always several central teeth, one lateral and many small 
marginals; and in the latter the typical radula, which is very long, has one or two 
pairs of central teeth, a large single lateral on each side, with a few small 
marginals. The first family ( Helicenidce ) forms a numerous group of small 
operculated land-shells, which abound in the West Indies and the islands of the 
Pacific and Indian Oceans. A few occur in Australia, China, and Central and 
South America, but they are unknown in Europe or Africa. Many are attractively 
coloured, and a few remarkable for the serrate keel at the periphery. Next come 
the nerites (Neritidce), all of which are aquatic, and feed upon vegetable substances. 
In Nerita the shells are globular, and strongly made to resist the action of the 
waves, for these molluscs are inhabitants of the seashore. They are furnished 
with a shelly operculum, which has a process jutting out from beneath and fitting 
under the toothed or wrinkled columellar lip when the mollusc retires within its 
shell. The species of Neritina have mostly thinner shells, 
especially those which inhabit fresh-water streams, and are also 
furnished with an operculum, like Nerita, but thinner. The 
pillar-lip is thin and smooth, or only finely dentate at the edge. 
In the section Clithon the shells are beset with a coronet of 
spines. The most remarkable form of that group is N. longi- 
spina, from mountain streams in the Mauritius. About two hundred species of 
Neritina are known, which abound in intertropical regions and the islands of the 
Pacific. One small species, N. fiuviatilis, occurs in Britain, where it is found in slow 
rivers with a stony or gravelly bottom, and is often coated with a calcareous deposit. 
The animal has a stout proboscis, long, pointed tentacles, and eyes placed upon short 
stumps at the base of the tentacles. The species of Septaria are somewhat limpet¬ 
like, but with the apex of the shell bent towards one end. They also have an 
operculum of a peculiar type, partly embedded in the foot. The species are prin¬ 
cipally met with in tropical islands. Among the fossil forms, Velates conoideus is 
interesting on account of its exceptional mode of growth. The top-shells ( Turbinidce ), 
like those of the next family, have one characteristic in common, namely, a bril¬ 
liantly pearly layer beneath the outer calcareous surface. The animals of both 
groups are vegetable-feeders and much alike, and are peculiar on account of the 
tentacular processes on the sides of the foot. In the Trochidce the operculum is 
horny, circular, multispiral, and with a central nucleus; in the Turbinidce, it is 
thickened with an outer shelly layer, consists of fewer whorls, and often has the 
nucleus excentric. The latter family is typified by the genus Turbo, which has been 
divided into a number of groups or subgenera on account of differences in this 
structure. The species are fairly numerous in tropical seas, but rare in more tem¬ 
perate regions. Of the allied genus Phasianella only one species (P. pullus ) reaches 
the British shores. The Trochidce have a wider range, are far more numerous, and 
occur everywhere from the Arctic to the Antarctic circles. The beauty of sculpture 
and coloration of many of the species of the typical genus Trochus is beyond descrip¬ 
tion, and can only be appreciated by an examination of the shells themselves. Most 
forms are littoral, or inhabitants of shallow water, but a few, and these among the 
most beautiful, have been dredged at enormous depths. Nearly twenty different 
species occur around the British coast, and some of them are extremely elegant in 
