Prof. Owen on the genus Dicynodon. 81 



pact arrangement of the calcigerous tubes of the dentine ; by which character it 

 makes a closer approach to the intimate texture of that tissue in the canine teetli 

 of the carnivorous MammaUa. I have shown, however, in my ' Odontography*,' 

 that the teeth of the Crocodilian Reptiles which are implanted in sockets have the 

 same essential type of structure as the simple teeth of the carnivorous Mammals, 

 The difference consists of slight and secondary modifications of the minute tubular 

 structure, and these disappear in the very remarkable teeth of the ancient extinct 

 Sauria under consideration. 



In the other Reptilia, recent or extinct, which most nearly approach the Mam- 

 malia in the structure of their teeth, the difference characteristic of the inferior and 

 cold-blooded class is manifested in the shape, and in the system of shedding and 

 succession, of the teeth : the base of the implanted teeth seldom becomes consoli- 

 dated, never contracted to a point, as in the fangs of the simple teeth of Mam- 

 malia, and at all periods of growth, one or more germs of teeth are formed within 

 or near the base of the tooth in use, prepared to succeed it, and progressing 

 towards its displacement. The dental armature of the jaws is kept in serviceable 

 order by uninterrupted change and succession ; but the matrix of the individual 

 tooth is soon exhausted, and the life of the tooth itself may be said to be com- 

 paratively short. 



Hitherto I have not failed to obtain evidence of this low organized dental con- 

 dition, — common to Fishesf, — in every Reptile, recent and extinct, in which I 

 could examine the implanted base of the teeth. 



The existing Lacertians superadd to this endless shedding and succession of 

 teeth, the ichthyic character of anchylosis of the base of the teeth in use to the 

 osseous substance of the jaw ; so that in the Rhynchocephalus and other Acrodont 

 Lizards j, the teeth appear like small enamelled processes of the alveolar border. 

 The Dicynodons not only manifest the higher type of free implantation of the 

 base of the tooth in a deep and complete socket, common to Crocodilians, Mega- 

 losaurs and Thecodonts, but make an additional and much more important step 

 towards the Mammalian type of dentition by maintaining the serviceable state of 

 the tusk by virtue of constant renovation of the substance of one and the same 

 matrix, according to the principle manifested in the long-lived and ever-growing 

 tusks and scalpriform incisors of the Mammalia. This endowment of the teeth of 

 a Reptile is far more remarkable and unexpected than the more obvious character 

 of the size and shape of the long exserted tusks themselves, superadded as they 

 are, and in such strange combination, with the otherwise edentulous jaws of a 



* Part II. pp. 281, 291. 



t Anomalous rostral teeth of Pristis and compound dental masses of Chimaroids excepted. 



X Odontography, p. 182. 



