320 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



teetli in rows upon the palatine, pterygoid and mandibular bones. 

 To complete the resemblance between the tasks of the Dicynodon 

 and the venom fangs of the snake, you must deeply groove their 

 fore-part, or bore a canal through their centre ; you must remove 

 those strong columns of bone which converge to, abut against, and 

 strengthen the fixed socket of the tusk, and you must suspend the 

 maxillary bone by a moveable pedicle to the pre-frontal and malar 

 bones. Besides, the perforated tusk of the poisonous serpent is 

 always followed by one or more similar teeth, in various stages of 

 growth, ready to supply its place, according to the general law of 

 the maintenance in serviceable state of the dental armature of the 

 jaws throughout the Reptilian class. 



The canine tusk of the Dicynodon consists of a simple body of 

 compact unvascular dentine, with a very thin outer coat of 

 enamel, which may be traced into the alveolus for a short dis- 

 tance. Rather more than one-third of the tusk is lodged in the 

 socket, the basal conical pulp-cavity is continued from the base 

 about one-half down the implanted part of the tusk, and a linear 

 continuation extends along the centre of the rest of the tusk, 

 from which the dentinal tubes of the solid body of the tusk radiate. 

 They present gentle parallel secondary curves or undulations 

 throughout their course, divide dichotomously 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 croco- 

 dile, consists in the closer and more compact 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 

 teeth of the carnivorous Mammalia. 



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

 approach the Mammalia in the structure of their teeth, the dif- 

 ference characteristic of the inferior and cold-blooded class is ma- 

 nifested in the shape, and in the system of shedding and succession 

 of the teeth. The dental armature of the jaws is kept in ser- 

 viceable 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 comparatively short. Evidence 

 of this low organised dental condition, common 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 teetli in use to the osseous substance of the jaw ; so 

 that in the Rhynchocephalus and other acrodont lizards, 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, Megalosaurs, and Thecodonts, but make an additional 



