FRONT-GILLED GROUP 3365 



SUBORDER Scutibranchiata 



The great feature of these animals is the absence of certain functional organs 

 in the different sexes; the radula being also of a peculiar type, and armed with sev- 

 eral central and lateral teeth. The two sections of the suborder, Rhipidoglossa 

 and Docoglossa, are based upon differences in the radula. In the 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 colored, 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 mollusks are inhabit- 

 ants 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 columel- 

 lar lip when the mollusk 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 FRESH-WATER NER- 

 with a coronet of spines. The most remarkable form of that IT E, Neritina. 

 group is N. longispina, 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 toward one end. They also have an operculum of a peculiar type, partly em- 

 bedded in the foot. The species are principally 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 brilliantly pearly layer beneath the outer cal- 

 careous 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 nu- 

 cleus; in the Turbinidce, it is thickened with an outer shelly layer, consists of fewer 

 whorls, and often has the nucleus eccentric. The latter family is typified by the 

 genus Turbo, which has been divided into a number of groups or subgenera on ac- 

 count of differences in this structure. The species are fairly numerous in tropical 

 seas, but rare in more temperate regions. Of the allied genus Phasianella only one 



