NUMBER AND ARRANGEMENT OF TEETH 2\J 



Genera which ar normHlly vegetarian, but which will, upon 

 occasion, eat fie. i, e.g. Limax and Hyallyiia^ 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 atlunticus about 21, in Fiona 7iohilis about 28. In 

 the common whelk (^Buccinum undatum) there are from 220 to 

 250, in the common periwinkle about 3500. As mau}^ as 8343 

 have been counted in Limnaea stagnalis, about 15,000 in JTelix 

 aspersa (thdx is, about 400,000 to the squ'^-'re inch), about 30,000 

 in Limax maximus, and as many as '^:0,000 in Helix Ghies- 

 breghti^ a large species from Mexico: they are very numerous 

 Iso in Nanina^ Vitrina^ G-adinia, anc«. Actaeon. But Umbrella 

 cands far and away the first, as far as number of teeth is con- 

 vdrned. In both U. mediterranea and U. indica they entirely 

 baffle calculation, possibly 750,000 may be somewheje 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 eve^-y 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 longi- 

 tudinally — 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 dis- 

 tinction between the laterals and marginals is fairly well indi- 

 cated, 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. 



The extreme importance of a study of the radula depends 

 upon the fact, that in each species, and a fortiori in each genus 



