It must not be forgotten, however, that a simple repetition of the same tooth, whether lateral or marginal, does not necessarily constitute an important characteristic, nor does the presence or absence of a central tooth. In most of the cases mentioned above, the difference in the number of laterals and marginals is due to the multiplication of identical forms, while the central tooth, when present, is often a mere plate or narrow block without cusps, whose presence or absence makes little difference to the character of the radula as a whole. There appear to be three well-marked types of radula among the Opisthobranchiata. (_a_) Radula with a single strong central tooth, rows few. This form is characteristic of the Aeolididae, Fionidae, Glaucidae, Dotoidae, Hermaeidae, Elysiidae (Fig. 135), and Limapontiidae. In the Aeolididae it is sometimes accompanied by a single lateral. The same type occurs in _Oxynoe_, and in _Lobiger_ (= _Lophocercus_). (_b_) Radula with the first lateral very strongly developed. This type may take the form of (1) a single lateral, no central or marginals, _e.g._ _Onchidoris_, _Scaphander_ (Fig. 137, A), _Philine_ (certain species), _Ringicula_, or (2) first lateral strongly developed, and repeated in succeeding laterals (2–6) on a smaller scale, _e.g._ _Philine_ (certain species). A few marginals are sometimes added, _e.g._ in _Polycera_, _Lamellidoris_ (where there is a degraded central tooth, Fig. 137, B), _Idalia_, and _Ancula_. [Illustration: FIG. 135.--Radula of _Elysia viridis_ Mont. × 40. Type (_a_).] [Illustration: FIG. 136.--Portion of the radula of _Gadinia peruviana_ Sowb., Chili. × 250. Type (_c_).] (_c_) Radula with an indefinite number of marginals, laterals (if present) merging into marginals, central tooth present or absent, inconspicuous, teeth all very small. This type of radula, among the Nudibranchiata, is characteristic of certain sub-genera of _Doris_ (_e.g._ _Chromodoris_, _Aphelodoris_, _Casella_, _Centrodoris_), of _Hypobranchiaea_ and _Pleurophyllidia_; among the Tectibranchiata, of _Actaeon_, many of the Bullidae, _Aplustrum_, the Aplysiidae, _Pleurobranchus_, _Umbrella_ and _Gadinia_ (Figs. 136 and 137, C). In the _Pteropoda_ there are two types of radula. The Gymnosomata, which are in the main carnivorous, possess a radula with a varying number (4–12) of sickle-shaped marginals, central tooth present or absent. In the Thecosomata, which feed on a vegetable diet, there are never more than three teeth, a central and a marginal on each side; teeth more or less cusped on a square base.