The number of teeth in the radula varies greatly. When the teeth are very large, they are usually few in number, when small, they are very numerous. In the carnivorous forms, as a rule, the teeth are comparatively few and powerful, while in the phytophagous genera they are many and small. Large hooked and sickle-shaped teeth, sometimes furnished with barbs like an arrow-head, and poison-glands, are characteristic of genera which feed on flesh; vegetable feeders, on the contrary, have the teeth rounded, and blunter at the apex, or, if long and narrow, so slender as to be of comparatively little effect. Genera which are normally vegetarian, but which will, upon occasion, eat flesh, _e.g._ _Limax_ and _Hyalinia_, exhibit a form of teeth intermediate between these two extremes (see Fig. 140, A). In _Chaetoderma_ there is but one tooth. In _Aeolis coronata_ there are about 17, in _A. papillosa_ and _Elysia viridis_ about 19, in _Glaucus atlanticus_ about 21, in _Fiona nobilis_ about 28. In the common whelk (_Buccinum undatum_) there are from 220 to 250, in the common periwinkle about 3500. As many as 8343 have been counted in _Limnaea stagnalis_, about 15,000 in _Helix aspersa_ (that is, about 400,000 to the square inch), about 30,000 in _Limax maximus_, and as many as 40,000 in _Helix Ghiesbreghti_, a large species from Mexico; they are very numerous also in _Nanina_, _Vitrina_, _Gadinia_, and _Actaeon_. But _Umbrella_ stands far and away the first, as far as number of teeth is concerned. In both _U. mediterranea_ and _U. indica_ they entirely baffle calculation, possibly 750,000 may be somewhere near the truth. The teeth on the radula are almost invariably disposed in a kind of pattern, exactly like the longitudinal rows of colour in a piece of ribbon, down the centre of which runs a narrow stripe, and every band of colour on one side is repeated in the same relative position on the other side. The middle tooth of each row--the rows being counted across the radula, not longitudinally--is called the _central_ or _rachidian_ tooth; the teeth next adjacent on each side are known as the _laterals_, while the outermost are styled _uncini_ or _marginals_. As a rule, the distinction between the laterals and marginals is fairly well indicated, but in the Helicidae and some of the Nudibranchiata it is not easy to perceive, and in these cases there is a very gradual passage from one set to the other. The central tooth is nearly always present. It is wanting in certain groups of Opisthobranchiata, some of the carnivorous Pulmonata, and in the Conidae and Terebridae, which have lost the laterals as well. _Voluta_ has lost both laterals and marginals in most of the species, and the same is the case with _Harpa_. In _Aeolis_, _Elysia_, and some other Nudibranchiata the radula consists of a single central row. Other peculiarities will be described below in their proper order.