32 THE LIFE OF THE MOLLUSCA 
They are sometimes numerous and all alike (Taxo- 
dont), especially in the more primitive Bivalves 
Nucula, Arca, etc. (Plate XVIL., Figs. 1, 3, 6, etc.). 
More usually they are divisible into a central, more 
or less transverse, group of “ cardinal teeth,” flanked 
on either side by others running with the shell 
margin, or “lateral teeth” (Teleodont). This shell 
margin bearing the ligament and teeth is known as 
the “ hinge line,” and is sometimes extended inwards 
so as to form a sort of platform, the “ hinge plate” 
(Plate XVI., Figs. 1 and 2). The number of teeth 
in the two valves differs, and occasionally, as in 
Chama, an individual will have the normal dentition 
of the two valves reversed, a condition apparently 
corresponding as near as such may be to sinistrosity 
in the Gastropod. The teeth become exceptionally 
strong in shells that live in situations exposing them 
to strain, and dwindle and disappear in such as 
dwell in protected localities. 
The possibility of the valves being laterally dis- 
placed is further guarded against in very many by 
tooth-like crenulations of the ventral margins of the 
valves that interlock, as in the Scallops, Cockles, 
etc. (Plate XV., Fig. 8; Plate XVIII., Fig. 3). 
The shelly tubes secreted by the Ship-worm 
(Tevedo, Plate XX., Fig. 5), Gastrochena (Plate XIX., 
Fig. 16), the Waterpot Shell (Brechites, Plate XX., 
Fig. 19), and some others to line their burrows are 
produced, it is true, by extensions of the mantle or 
