THE TEETH OF REPTILES. 243 



Vase-dentine also occurs in the teeth of some saurians, as 

 for example, in those of the great extinct Iguanodon. in 

 which it, roughly speaking, formed the inner half of the 

 crown, the outer moiety consisting of hard dentine. In 

 addition to this peculiarity, the teeth of Iguanodon were 

 remarkable for the partial distribution of the enamel, which 

 was strongly ridged, the ridges being serrated, and was con- 

 fined to the outer side of the crown. Thus at the outside 

 came the hardest tissue, the enamel; next the harder dentine 

 and on the inside, the softer vase-dentine. Hence, as the 

 tooth wore down, a sharp edge was long preserved. 



There is a New Zealand lizard, to which the several names 

 of Hatteria, Sphenodon, and Rhyncocephalus have been given, 

 which has a very peculiar dental armature (Dr. Giinther, 

 Phil. Trans., 1867). 



The inter-maxillary bones are armed with two teeth, so 

 large as to be co-extensive with the whole bone in width, 

 and of a form which recalls that of the gnawing incisors 

 of Rodents ; the other teeth are quite small, and " acrodont " 

 in their attachment. 



But the great peculiarity of Hatteria is that the alveolar 

 margins of the jaws are sharp, and when the teeth are worn 

 down, which would happen in adult specimens, the actual 

 sharp margins of the bone come into play as masticatory 

 organs, near to the front of the mouth. It occurred to me 

 as probable that the surface thus exposed might be coated 

 with dentine, but a microscopic examination of one of the 

 specimens in the British Museum, which I was, by the 

 kindness of Dr. Giinther, enabled to make, proved that the 

 dense ivory-like surface which serves the purposes of masti- 

 cation is true bone, and has no relation to dental structure. 



There are very few other instances of actual bone, un- 

 coated by dental tissues, being used for masticatory purposes. 



The great extinct Dicynodon, an African fossil, also 

 had sharp trenchant margins to its jaws ; it is not 



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