8 PKOCEBDINGS OF THE MALACOLOGICAL SOCIETY. 



laterals; in the greatmajority the rhachidian tooth alone remains. ^ 

 Even in the rare cases in which laterals are present, they are 

 markedly degraded, and, as it happens, in varying degrees. On the 

 analogy of the radula of the Mitridse,^ a family nearly related to the 

 Voliitid3e,the primitive Volutidan lateral was probably furnished with 

 a number of small sharp cusps. These, as in certain Mitra, have 

 coalesced, in V. concinna, into a single large, but obviously degraded 

 cusp, the base of which extends from one end of the framework to 

 the other. In Vol. dbyssicola further degradation has taken place, 

 the single cusp is smaller, and stands on the inner end of the frame- 

 work, the rest of the margin being bare. In Nc'pt. gilchristi the 

 laterals are greatly reduced in size, " probably quite functionless, 

 of extreme tenuity compared with the very massive rhachidian 

 tooth, and their contours are ragged " (Pace, 18). 



A similar process of modification appears to have taken place in 

 the case of the rhachidian tooth. Its primitive form, preserved in 

 V. musica alone, was probably multicuspid, and a like process of 

 coalescence has produced the normal tricuspid rhachidian. Even 

 in V. musica, the two external cusps are much larger than the others, 

 differently shaped, and bear a close resemblance to those of Lyria 

 and (see below) one or two Voluta proper (Fig. 1). The instability 

 of the smaller cusps in the rhachidian of F. musica is easily shown. 

 Excluding the two large side cusps, Pace^ (18) figures a tooth with 

 eleven cusps (ten large and one small), Fischer (" Manuel," p. 609) 

 one with thirteen (ten large, three small). The " Mus. Brit." 

 specimen in the Gwatkin collection has ten cusps at the nascent end 

 (eight large, two small), twelve in the middle (eight large, four 

 small), eleven near the front end.* 



With this exception, and one to be noted below, all the known 

 species of Melo, Cymbium, Voluta (including Lyria and Volutilithes, 

 but not Amoria, Halia, and Volutomitra) have a tricuspid rhachidian 

 (save where the tooth has vanished altogether). Here, however, we 

 may distinguish four groups : — 



Group A. — Cusps massive, long, and swordlike ; framework 

 thick, all deeply stained with red, brown, or orange (Fig. 2). To 

 this group belong the great mass of the species of Voluta, with all 

 the known species of Cymbium and Melo. 



Group B. — Cusps rather short, thin, transparent, somewhat far 

 apart ; colour light yellow ; the two outer cusps very broad at their 



^ Some day a Marginella with laterals will turn up. 



2 A. H. Cooke, Proc. Zool. Soc, 1919, pp. 405-22. 



' This author hints at the possible occurrence of " shapeless vestiges of 

 lateral teeth " in F. musica. 



* In the " Mus. Brit." specimen one of the additional cusps originates as 

 a denticle high up on the side of a cusp ; in the succeeding rows this denticle 

 gradually becomes larger and descends, until at last it disengages itself entirely. 

 In another instance the new cusp starts, from the first, as a tiny separate 

 denticle, and gradually becomes larger in succeeding rows. 



