The extreme importance of a study of the radula depends upon the fact, that in each species, and _a fortiori_ in each genus and family, the radula is characteristic. In closely allied species the differences exhibited are naturally but slight, but in well-marked species the differences are considerable. The radula, therefore, serves as a test for the distinction of genera and species. For instance, in the four known recent genera of the family Strombidae, _viz._ _Strombus_, _Pteroceras_, _Rostellaria_, and _Terebellum_, the radula is of the same general type throughout, but with distinct modifications for each genus; and the same is true, though to a lesser extent, for all the species hitherto examined in each of the genera. These facts are true for all known genera, differences of the radula corresponding to and emphasising those other differences which have caused genera to be constituted. The radula therefore forms a _basis of classification_, and it is found especially useful in this respect in dealing with the largest class of all, the Gasteropoda, and particularly with the chief section of this order, the Prosobranchiata. Thus we have-- { (_a_) _Toxoglossa_ { (_b_) _Rachiglossa_ { _Monotocardia_ { (_c_) _Taenioglossa_ { { (_d_) _Ptenoglossa_ =Prosobranchiata= { { (_e_) _Gymnoglossa_ { { _Diotocardia_ { (_f_) _Rhipidonlossa_ { (_g_) _Docoglossa_[324] (_a_) _Toxoglossa._--Only three families, Terebridae, Conidae, and Cancellariidae, belong to this section. There is no central tooth, and no laterals, the radula consisting simply of large marginals on each side. In _Conus_ these are of great size, with a blunt base which contains a poison-gland (see p. 66), the contents of which are carried to the point by a duct. The point is always singly and sometimes doubly barbed (Fig. 116). When extracted, the teeth resemble a small sheaf of arrows (Figs. 113, 115). A remarkable form of radula, belonging to _Spirotropis_ (a sub-genus of _Drillia_, one of the Conidae), enables us to explain the true history of the radula in the Toxoglossa. Here there are five teeth in a row, a central tooth, and one lateral and one marginal on each side, the marginals being very similar in shape to the characteristic shafts of the Conidae (Fig. 114). It is evident, then, that the great mass of the Toxoglossa have lost both their central and lateral teeth, and that those which remain are true uncini or marginals. _Spirotropis_ appears to be the solitary survival of a group retaining the primitive form of radula. [Illustration: FIG. 113.--Radula of _Bela turricula_ Mont. × 70.] [Illustration: FIG. 114.--Portion of radula of _Spirotropis carinata_ Phil., Norway. × 70.]