32 THE LIFE OF THE MOLLUSCA 



They are sometimes numerous and all alike (Taxo- 

 dont), especially in the more primitive Bivalves 

 Nucula, Area, etc. (Plate XVII., Figs, i, 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 XVIIL, Fig. 3). 



The shelly tubes secreted by the Ship-worm 

 (Teredo, Plate XX., Fig. 5), Gastrochcena (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 



