[Illustration: 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.] [Illustration: 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. [Illustration: Fig. 125.--Portion of the radula of _Cassis sulcosa_ Born., × 40. The marginals to the right are not fully drawn.]