DEVELOPMENT OF TEETH IN HATTERIA PUNCTATA. 205 



of growth of the animal there is a constant suc- 

 cession of these teeth from behind forwards, the 

 series having once commenced. 



11. All the teeth except those of the first dentition have 

 an outer coating of enamel, which, however, does not extend 

 as far as the base of the fully formed teeth. It contains 

 prolongations of the dentinal tubules. The odonto- 

 blasts of many of the teeth remain in activity for a con- 

 siderable period, probably at least till the animal is full- 

 grown. 



12. The teeth of the very young animal arc typically 

 acrodont in their attachment. Later, through a secondary 

 formation of bone round their bases, many of them come to 

 lie in shallow alveoli, apparently similar to those described 

 in some fossil Rhynchocephalians, such as Proterosaurus and 

 Champsosaurus. Since the teeth are firmly fused with the 

 base and sides of the alveoli, we are not justified in calling 

 them thecodont, and the condition is best described by the 

 use of the term 'Miyperacrodont," suggested by Howes and 

 Swinnerton. 



The sides of the jaws are covered with a specialised layer 

 of bone, apparently more highly calcified than the rest. In 

 the adult a large part of the jaw is exposed, and when the 

 teeth are worn down this outer layer assists the exposed 

 portion to functionally replace the teeth. In some of its 

 macroscopic and microscopic features it resembles enamel. 



13. The evidence of the tooth development and 

 succession suggests that the long incubation 

 period of Hatteria (thirteen months) is a compara- 

 tively recently acquired character.^ This view affords 

 us an explanation of the possession, after about five months 

 of incubation, of a complete (though degenerate) dentition, 



^ This view is also supported by several of Deudy's discoveries. The 

 presence of a characteristic pattern on the skin, after about five months' 

 incubation, and its almost complete disappearance before hatching, is very 

 suggestive. So also is the very early development of the epidermal shell- 

 breaker on the snout. 



