58 
PHYLLIVOKA. 
surface of the plates, but are abruptly curved pos- 
teriorly, so that the apices of the projection inva- 
riably point towards the closed end of the tongue, 
£ 19. and 20. These teeth are distributed in rows all 
over the membrane, and are closely packed together. 
The longitudinal rows always consist of straight 
lines, but the transverse rows are variously curved, 
often bow-shaped, sometimes angular, rarely straight 
(fig. 18 .). 
Fig. 18 . 
a 
b 
Direction of the cross series of Teeth in 
a, Planorbis contortus. h, Ancylus fluviatilis. 
c, Zonites radiatulus. Achatina acicula. 
e, Zua lubrica. f Vitrina pellucida. 
g, Limax carinatus. A, Helix obvoluta. 
The degree of curvature of the transverse row, 
and the variations which the curves show (being 
sometimes composed of arcs of circles, while at 
other times they are made up of short straight lines 
