2 I 8 VALUE IN CLASSIFICATION 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, JPteroceras, Bostellaria, and Terebellum, 

 the radula is of the same general type throughout, but with dis- 

 tinct 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, differ- 

 ences of the radula corresponding to and emphasising those other 

 differences which have caused genera to be constituted. The 

 radula therefore forms a hasis 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 — 



'(«) Toxoglossa 



(b) EacJiiglossa 



Monotocardia A (c) Taenioglossa 



(d) Ftenofjlossa 



(e) Gymnoglossa 



Diotocanlia {'^^ Rhipidoglossa 

 yifj) Docoglossa ^ 



Prosobranchiata 



(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. 1 13, 115). A re- 

 markable form of radula, belonging to Sjpirotropis (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 character- 

 istic shafts of the Conidae (Fig. 114). It is evident, then, that 

 the great mass of the Toxoo-lossa have lost both their central 



^ t6^ov, arrow ; pax's, ridge, sharp edge ; raipia, ribbon ; Trrrivbs, winged ; yv/j.v6s, 

 bare ; piTris, fan ; 5ok6s, beam. 



