[Illustration: FIG. 115.--Eight teeth from the radula of _Terebra caerulescens_ Lam. × 60.] The arrangement of teeth in all these sections is expressed by a formula applicable to each transverse row of the series. The central tooth, if present, is represented by 1, and the laterals and marginals, according to their number, on each side of the central figure. Thus the typical formula of the Toxoglossa is 1.0.0.0.1, the middle 0 standing for the central tooth which is absent, and the 0 on each side of it for the absent laterals; the 1 on each extreme represents the one uncinus in each row. Thus the formula for _Spirotropis_, which has also one lateral on each side and a rachidian or central tooth, is 1.1.1.1.1. Often the formula is given thus: 1.0.0.0.1 / 30, 1.1.1.1.1 / 42, where 30 and 42 stand for the average number of _rows_ of teeth in _Conus_ and _Spirotropis_ respectively; the same is sometimes expressed thus: 1.0.0.0.1 × 30; 1.1.1.1.1 × 42. [Illustration: FIG. 116.--A tooth from the radula of _Conus imperialis_ L., S. Pacific, × 50, showing barb and poison duct.] [Illustration: FIG. 117.--Portion of the radula of _Melongena vespertilio_ Lam., Ceylon. × 30.] [Illustration: FIG. 118.--Portion of the radula of _Eburna japonica_ Sowb., China. × 30.] [Illustration: FIG. 119.--Portion of the radula of _Murex regius_ Lam., Panama. × 60.] (_b_) The _Rachiglossa_ comprise the 12 families Olividae, Harpidae, Marginellidae, Volutidae, Mitridae, Fasciolariidae, Turbinellidae, Buccinidae, Nassidae, Columbellidae, Muricidae, and Coralliophilidae. Certainly most and probably all of these families are or have been carnivorous, the Coralliophilidae being a degraded group which have become parasitic on corals, and have lost their teeth in consequence. The characteristics of the group are the possession of a central tooth with from one cusp (_Boreofusus_) to about fourteen (_Bullia_), and a single lateral more or less cuspidate, the outer cusp of all being generally much the largest. Thus in _Melongena respertilio_ (Fig. 117) the central tooth is tricuspid, the central cusp being the smallest, while the laterals are bicuspid; in _Eburna japonica_ (Fig. 118) the central tooth is 5-cusped, the two outer cusps being much the smallest. The teeth, on the whole, are sharp and hooked, with a broad base and formidable cutting edge. In the Olividae, _Turricula_, _Buccinopsis_, and the Muricidae the laterals are unicuspid and somewhat degraded (Fig. 119). In _Mitra_ and the Fasciolariidae they are very broad and finely equally toothed like a comb (Figs. 120, 121). The whole group is destitute of marginals. [Illustration: FIG. 120.--Portion of the radula of _Imbricaria marmorata_ Swains. × 80.] [Illustration: FIG. 121.--Three rows of teeth from the radula of _Fasciolaria trapezium_ Lam. × 40.] [Illustration: FIG. 122.--Six teeth from the radula of _Cymbium diadema_ Lam., Torres Strait. × 25.]