80 Prof. Owen on the Reptilian Fossils of South Africa. 



of the true Saurian character of the Dicynodonts, by the close resemblance which 

 they offer to the structure of the teeth of the Crocodile, Teleosaur and Plesiosaur. 



The canine tusk of the Dicynodon consists of a simple body of compact unvas- 

 cular dentine, with a very thin outer coat of enamel, which may be traced into the 

 alveolus for a short distance. 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 hnear continuation extends along the 

 centre of the rest of the tusk. From this central line the dentinal tubes of the solid 

 body of the tusk radiate ; with a gentle curve at their beginning, convex towards 

 the point of the tusk, and then proceeding straight to the periphery of the tooth ; 

 not quite vertically to that surface, but slightly inclined downwards, or towards 

 the apex. They present gentle parallel secondary curves or undulations through- 

 out 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. At their primary curve, near the centre of the tooth, the dentinal 

 tubes measure iwooth of an inch in diameter, and are separated by intervals of 

 double that breadth, or eooo th of an inch. They diminish in size as they approach 

 the periphery of the dentine and terminate close to the enamel by dividing or 

 breaking up into minute irregular ramuli. The dentinal cells are most conspicuous, 

 as in the teeth of the Crocodile*, near the periphery of the tooth ; they are sub- 

 hexagonal, and vary in diameter from g-^oth to a xcuroth of an inchf. The stratified 

 arrangement of these cells in planes parallel with the surface of the tooth is very 

 conspicuous, as in fossil teeth generally, from the partial decomposition of the 

 basal tissue of the dentine. To the naked eye, or with a low power, this disposition 

 of the dentinal cells gives the appearance of a concentrically-laminated structure of 

 the dentine ; the more highly organized and characteristic tubular structure de- 

 manding higher powers for its detection. The enamel, at least at the middle of the 

 tusk, where I have examined it, is thinner than in the teeth of the Crocodile ; it 

 presents only a finely lamellated texture, the layers being parallel with the surface 

 of the dentine on which it rests. There is only a fine linear trace of cement on the 

 exterior of the sections of the implanted base of the tusks ; and here it is too thin 

 to allow of the development of the radiated cells in its substance. 



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 com- 



* See Report of British Association, 1838, p. 144. 



t The observations on the dental tissues of the Dicynodon were made both with my own (Smith's) 

 microscope and with the splendid instrument with screw-micrometer (Powell's), in the possession of my 

 friend C. Stokes, Esq., for whose kind aid in these and similar microscopic inquiries I beg to express my 

 grateful acknowledgements. 



