GASTROPODS. 



395 



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 (Neritidw), 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. 



1 L J ^ & FRESHWATER NERITE, 



Iii the section Clithon the shells are beset with a coronet of Neritina (nat. siz L -). 

 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. fluviatilis, 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 (Turbin'uh i ), 

 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 Twrbinidce, 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 Phasiandla only one species (P. pullus) reaches 

 the British shores. The Trochida 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 



