148 JOURNAT. OF CONCHOLOGY, VOL. 16, NO. 5, JANUARY, I92I. 



laterals, and its cusps weaker; the cusps of the laterals are often 

 powerful sabre-like weapons, and, although never barbed, are some- 

 times hooked at the apex {Oliva). 



The most primitive radula-form in this group is that of the Fascio- 

 lariidce, Fusmcs, and MitridcB, where the laterals are broad, comb- 

 shaped, and multicuspid, while the median tooth is small, and, as a 

 rule, weakly armed. Here, as so often, increased power in one 

 member of. the row seems to be accompanied by the weakening of 

 another member, or by its complete extinction. Thus Valuta and 

 Marginella lose both laterals and marginals ; sometimes the process 

 is reversed, and, as seen in the Toxoglossa, medians and laterals dis- 

 appear, while marginals increase in size ; sometimes {e.g., in the 

 FusincE) intermediate stages are seen, when powerful laterals have 

 almost extinguished the weak and in some cases scarcely discernible 

 median tooth. Sometimes (certain species oi Mitra) the laterals are 

 denuded of cusps, while the cusps of the median tooth are strengtii- 

 ened.^ The same principle, that an organism economises energy, 

 saves on one part what it gains on another, is seen in the land 

 Agnatha, e.g., in Rhytida krmissi Pfr., from the Cape, where a huge 

 development of the external marginal is accompanied by a marked 

 weakening of the other teeth in the row. 



In the Rhachiglossa we have further indication that homologous 

 parts tend to cohere, that large cusps are formed by the coalescence 

 of small ones. Alectrion retains a median tooth with numerous small 

 cusps, while in many .species of the allied genus PIios the cusps in 

 the median have fused into three. Vexillum (formerly Turricula) 

 falls into two groups, in one of which numerous very small cusps arm 

 the median tooth : in the other group all these have coalesced into 

 three, placed in the centre of the margin. Single-cusped laterals 

 are probably due to the fusion of two, three, or more cusps in earlier 

 forms. 



Certain cases occur in which, for reasons at present unknown, the 

 teeth are unciform, suggesting carnivorous habits, but their number is 

 greatly multiplied, with no definite median tooth, and no distinction 

 between laterals and marginals. This form of radula is illustrated 

 by Epitoniuin {Scalaria) and lanthina, and recalls, in general appear- 

 ance, the radula of Testacella. Some one ought to find out on what 

 "laiitliina feeds. Is it the Vetella with which it associates ? 



Perhaps, of all the radulae known to us, that of the Volutidce. is the 

 most interesting, illustrating as it does the process of evolution 



1 A. H. Cooke in Proc. Zool. Soc, 1919, pp. 405—422. 



2 Since writing the above, I find that Jefifreys (Brit. Conch., iv, p. 176) has collected 

 al)iindant evidence that lanthina is carnivorous, feeding on Velelia, other lanihina, Phjsalia, 

 Porpita, and Lepas. 



