Fig. 121.—Three rows of teeth from the radula of Fasciolaria trapezium Lam. × 40.
Fig. 122.—Six teeth from the radula of Cymbium diadema Lam., Torres Strait. × 25.
Fig. 123.—Examples of degraded forms of radula: A, Cantharus pagodus Reeve, Panama (nascent end), × 40; A´, same radula, central and front portion; B, Columbella varia Sowb., Panama, × 50.
Fig. 124.—Three rows of the radula of Sistrum spectrum Reeve, Tonga, × 80. The laterals to the right are not drawn in.
Several remarkable peculiarities occur. Harpa loses the radula altogether in the adult. In the young it has lost only the laterals, and consists of nothing but the central tooth. Marginella has no laterals; the central tooth is small and comb-shaped, with blunt cusps. In Voluta the laterals are generally lost, but in Volutomitra and one species of Voluta[325] they are retained. The central tooth usually has three strong cusps, and is very thick and coloured a deep red or orange (Fig. [122]); in the sub-genus Amoria it is unicuspid, in shape rather like a spear-head with broadened wings; in Volutolyria it is of a different type, with numerous unequal denticulations, something like the laterals of Mitra or Fasciolaria. Of the Mitridae, Cylindromitra has lost the laterals. Among the Buccinidae, Buccinopsis possesses a curiously degraded radula, the central tooth having no cusps, but being reduced to a thin basal plate, while the laterals are also weakened. This degradation from the type is a remarkable feature among radulae, and appears to be characteristic, sometimes of a whole family, e.g. the Columbellidae (Fig. [123], B), sometimes of a genus, sometimes again of a single species. Thus in Cantharus (a sub-genus of Buccinum) the radula is typical in the great majority of species, but in C. pagodus Reeve, a large and well-grown species, it is most remarkably degraded, both in the central and lateral teeth (Fig. [123], A). This circumstance is the more singular since C. pagodus lives at Panama side by side with C. ringeus and C. insignis, both of which have perfectly typical radulae. It is probable that the nature of the food has something to do with the phenomenon. Thus Sistrum spectrum Reeve was found to possess a very aberrant radula, not of the common muricoid type, but with very long reed-like laterals. This singularity was a standing puzzle to the present writer, until he was fortunate enough to discover that S. spectrum, unlike all other species of Sistrum, lives exclusively on a branching coral.
The dental formula for the Rachiglossa is thus 1.1.1, except in those cases where the laterals are absent, when it is 0.1.0.
