Mr Owen on the Dici/nodon. 337 



twice or thrice near their beginnings, and send off numerous 

 small lateral branches, chiefly, but not exclusively, from the 

 side next the apex. 



The principal difference in the microscopic texture of the 

 tusks of the Dicynodon, as compared with the teeth of the 

 crocodile, consists in the closer and more compact arrange- 

 ment of the calcigerous tubes of the dentine ; by which cha- 

 racter it makes a closer approach to the intimate texture of 

 that tissue in the canine teeth of the carnivorous Mammalia. 



In the other Reptilia, recent or extinct, Avhich most nearly 

 approach the Mammalia 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 dental armature of the jaws is 

 kept in serviceable order by uninterrupted change and succes- 

 sion ; but the matrix of the individual tooth is soon exhausted, 

 and the life of the tooth itself may be said to be comparatively 

 short. Evidence of this low organised dental condition, com- 

 mon to fishes, has been obtained in every reptile, in which 

 the implanted base of the teeth has been examined by the 

 author. 



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 Bhynchocephalus and other acrodont lizards, 

 the teeth appear like small enamelled processes of the alveo- 

 lar 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, Megalosaurs, 

 and Thecodonts, but make an additional and nnich more im- 

 portant .step towards the Mammalian type of dentition by 

 maintaining the serviceable state of the tusk by virtue of con- 

 stant renovation of tlie substance of one and tlie same matrix, 

 according to the principle manifested in the long-lived and 

 ever-growing tusks and scalpriform incisors of tlie IMammalia. 

 This endowment of the teeth of a reptile is far more remark- 

 able and unexpected than the more obvious character of the 

 size and shape of the long exserted tusks themselves, super- 

 added as they are, and in such strange combination, with tlie 



VOL. XXXIX. NO. /iXXVIH. — OCTOBKR 1845. Y 



