COOKE : EVOLUTION IN THE MOLT.USCAN RADULA. I49 



which has gone on in that genus, and enabhng us to infer by analogy 

 what may have happened in other genera. 



Nearly all the known Volutidce {Cymbium, Melo, Vohtta, Lyria, 

 Avioria, Vohitoinitra, Halia) have a radula consisting of a single 

 strong tricuspid median, with no laterals or marginals. G. Schacko 

 discovered^ that a lateral is present in V. concinna Brod., consisting 

 of a single large triangular cusp covering a broad base. 



A further stage back is seen in Voliitilithes abyssicola Ad. and Rve., 

 in which the single cusp is placed at one end of the oblong lateral, 

 from the rest of which must have perished the other cusps which 

 once covered the margin, as in the parallel case of certain of the 

 Mitridce. In yet another species {Neptzitieopsis gilchristi ^o^h)) Pace 

 has found - traces of a vestigial lateral, cuspless, of a highly tenuous 

 and degraded character. Further, it can hardly be doubted that in 

 the multicuspid median of V. musica L. we have a survival of a more 

 ancient form of volutidan radula, before the small cusps on that tooth 

 cohered into the strong tricuspid form, but in which the laterals seem 

 to be entirely lost. 



A further stage of progression is seen in the groups Anioria and 

 Volutoinitra. Here the tricuspid median has become unicuspid, 

 while the ' wings ' of the framework are greatly prolonged. In Halia 

 the radula is very small, and very narrow in proportion to its length, 

 with a small median, shaped as in Amoria, and a doubtful lateral on 

 each side ; probably a case of general degradation. Finally, four 

 species of Voluta (do hrn i ^ov^h., dubia Brod. , gou/dzana \)d\\ junonia 

 Lam.) have, according to Dall,^ lost the radula altogether. 



Degradation of the Radula. 



In certain groups of Marine Gasteropoda, the complete absence or 

 radula has occasioned their classification as ' Gymnoglossa.' The 

 title is unfortunate, because the group as constituted does not include 

 all Gasteropoda destitute of radula. It is perhaps best to regard 

 those instances which occur, as the last terms in a series of gradual 

 degradation, due in many cases to known eccentricities of habitat. 

 Thus no radula occurs in Euii/na, some species of which are known 

 to be parasitic or commensal on Holothurians, or in Stilifer and 

 Styliferina, which are parasitic on or in or commensal with Echino- 

 derms and Holothurians. Some of the Pyraniidellidce. {Odostoniia) 

 are commensal with Pecien. 



1 Conch. Mittheil., i (ii) 1881, pp. 122 — 128, pi. 24, f. 5. 



2 Proc. Malac. Soc, v, 1903, pp. 21 — 30, pi. 2, f. 8. 



3 Bull. Mus. C.Z. Harv., xviii, 1889, pp. i — 492 : Smiths, Misc. Coll., 48, 1907, pp. 341—373. 



