PHYLLIVORA. 



61 



Fig. 23. 



it presents the form of an Inverted tapering triangle, 

 with a rounded apex (^Amphipeplea). 



" The form of the tubercle on the central plate is 

 subject to much greater variation than its plate. 

 Sometimes the tubercle is 

 very large and attached 

 to nearly the whole sur- 

 face of the plate, leaving 

 but a small free apex {Li- 

 max, fig. 21.). In other 

 species the tubercle is 

 small, and attached by its 

 base to the anterior por- 

 tion of the plate {Zua, 



fig. 23. a). In another genus {Planorbis) we find that 

 the tubercle is small, and has two apices (fig. 24. a). 



Teeth of Zua lubrica. 

 a, central; b, first lateral tooth. 



Fig. 24. 



Teeth of Planorbis caiivatus. 

 a, central ; b, first ; c, intermediate ; d, outer lateral tooth. 



The apex in some few instances projects beyond the 

 edge of the plate (fig. 21. a.), and consequently lies 

 above the base of the tooth next beyond it ; but, in 

 the majority of cases, the apex of the central tubercle 

 does not project over the edge of its plate (fig. 21. a). 

 " The lateral plates not only differ from the cen- 



