V.J 



THE COMMON FROG. 



section shows that these surface-grooves are the ex- 

 ternal indications of deep indentations of the substance 

 of the tooth. All these indentations converge towards 

 the centre of the tooth, but not in straight lines, each 

 indentation being elaborately inflected. Radiating 



Fig. 23. — Much enlarged horizontal section of the tooih of a Labyrinthoiloi,t. 



from the centre of the tooth we find a corresponding 

 number of processes of the central pulp cavity — the 

 radiating process undulating like the converging folds. 

 A similar structure of tooth is found in some Ganoid 

 fishes, and an incipient stage (as it were) of the same 

 condition existed in the Ichthyosaurus. 



