58 



PHYLLIVORA. 



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, 

 f. 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. 



Direction of the cross series of Teeth in 



a, Planorbis contortus. h, Ancylus fluviatilis. 



c, Zonites radiatulus. d, Achatina acicula. 



e, Zua lubrica. /, Vitrina pcllucida. 



(J, Limax carinatus. h. Helix obvoluta. 



" The decree 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 



