VIII NUMBER AND ARRANGEMENT OF TEETH 2 1/ 



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 Glaiicus atlanticus about 21, in Fiona noMHs about 28. In 

 the common whellv (Btcccimim undatum) there are from 220 to 

 250, in the common periwinkle about 3500. As many as 8343 

 have been counted in Limnaea stagncdis, about 15,000 in Helix 

 o,spersa (that is, about 400,000 to the square inch), about 30,000 

 in Limax maximus, and as many as 40,000 in Helix Ghies- 

 hreghii, 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 con- 

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

 battle 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 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 



