12 THE LIFE OF THE MOLLUSC A 



such case being more quickly worn away by use. 

 The worn-out teeth usually fall off and are lost, but 

 in one tribe (Ascoglossa = Elysioidea) they are received 

 and retained in a special sac. 



The total number of teeth in the radula conse- 

 quently varies greatly in the different kinds. In 

 some species of Chcetoderma there is but a single 

 tooth present, while the Whelk (Plate III., Figs. 16 

 and 17) has about 250, a big Limpet 2,000, the 

 Periwinkle 3,500, the Common Garden Snail 15,000, 

 and the big Grey Slug (Limax maximus) 26,800. 

 In certain instances they almost baffle calculation, 

 as many as 750,000 being computed for Umbra- 

 culitm. 



Since each succeeding row of teeth resembles its 

 predecessor, a longitudinal striped pattern results 

 when the teeth in the transverse rows vary in shape 

 and colour. In each row there is generally a central, 

 or " rhachidian," tooth present, flanked on either side 

 by " laterals." These laterals may be of one type, 

 as in the Common Garden Snail and its allies 

 {Helix), or they may be divisible into two or three 

 sets, when those nearest the central tooth are styled 

 " admedian " and the outer ones " marginal," these 

 two series being generally divided by a conspicuous 

 tooth, or " major lateral." This is expressed by 

 formulae, as 1:1:1 (Plate III., Figs. 16 and 17), 

 2:1:1:1:2 (Plate III., Fig. 15), and when the 

 marginal teeth are very numerous, 00 : 5 : 1 : 5 : oo 



