2l8 VALUE IN CLASSXFICAIION chap. 



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 Terebel- 

 lum^ 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 geneip, 

 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 — 



Prosobranchiata \ 



r (a) Toxoglossa 



I (p) Rachiglossa 



Monotocardia \ (c) Taenioglossa 



(d) Ptenoglossa 



(e) Gymnoglossa 



\ ^. ,. f Cf) Rhipidoqlossa 



Diotocardia i ) x 7^ ? 1 

 [ [ {g) Docoglossa '■ 



^ 



(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 mar- 

 ginals on each side. In Conus these are of great size, with a 

 blunt base which contains a poison-gland (see p. QQ^)., the con- 

 tents 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 subgenus 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 



1 t6^ov^ arrow ; pdxis, ridge, sharp edge ; raivla, ribbon ; irTrjvdi, winged ; 

 yv/xvSs^ bare ; piTrls, fan ; 5o/c6s, beam. 



