PHYLLIVORA. 
61 
it presents the form of an inverted tapering triangle, 
with a rounded apex (^Amphipepled). 
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 ^eeth of Zua lubrica. 
base to the anterior por- a, central J b, first lateral tooth. 
tion of the plate {Zua^ 
fig. 23. a). In another genus (^Planorhis) we find that 
the tubercle is small, and has two apices (fig. 24. d). 
Fig. 24. 
Teeth of Planorbis carinatus. 
a, central ; b, first ; c, intermediate ; (/, 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- 
