12 THE LIFE OF THE MOLLUSCA 
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) theyare 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 Chetoderma 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 Umobra- 
culum. 
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 
formule, 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, © :5:1:5:@ 
