THE LAMELLIBRANCHIA 



59 



makes possible a very convenient system of symbols, 

 for which we are indebted to the French palaeontologists 

 Munier-Chalmas and Bernard. The lateral teeth are 

 numbered from within outwards, so that those of the 

 right valves have odd numbers, those of the left even : 

 thus the right anterior cardinals are Ai. and Am., the 



FIG. 16. HETERODONT HINGES. 



Diagrammatic representation of Cyrenoid type (above) and Lucinoid type 

 (below). Interior views of right valves, all possible teeth represented, 

 and those originating from the same lamina shown as connected. 

 Teeth of right valve, black; sockets corresponding to teeth of left 

 valve, dotted. 



left An. and Aiv. ; the posterior-laterals Pi. and Pin., 

 PH., and Piv. The symbols for the cardinals are 

 based upon the facts of early development, which show 

 them to be the detached hook-like ends of laminae of which 

 the main portion forms the anterior-laterals : thus the 

 right middle cardinal is derived from lamina i., and so is 



