400 ANATOMY OF VERTEBRATES. 



the open extremity of the socket ; the two converging, as they 

 descend along the outer side of the compressed symphysis of 

 the lower jaw. The tusk is principally composed of a body of 

 compact unvascular dentine. The base is excavated by a wide 

 conical pulp-cavity with the apex extending to about one half 

 of the implanted part of the tusk, and a linear tract is continued 

 along the centre of the solid part of the tusk. 



The enamel is thinner than in the teeth of the Crocodile. 

 There is a trace of cement on the exterior of the sections of the 

 implanted base of the tusks. In the lower jaw, ib. 25, 23, the 

 alveolar border of the dentary element presents a smooth and even 

 edge, which seems to have played like a scissor-blade upon the 

 inner side of the corresponding edentulous border of the upper 

 jaw. In some Dicynodonts (Ptyclioynatlius) the symphysis was 

 singularly produced upward. 



Until the discovery of the Rliynclwsaurus 1 and Oudenodon, this 

 edentulous and horn-sheathed condition of the jaws was supposed 

 to be peculiar to the Chelonian order among reptiles. 2 In the 

 Saurian Dicynodon we find, superadded to the horn-clad man- 

 dibles of the Tortoise, a pair of tusks, borrowed as it were from 

 the mammalian class, or rather foreshadowing a structure which, 

 in the actual creation, is peculiar to certain members of the 

 highest-organised warm-blooded animals. 



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 

 manifested in the shape, and in the system of shedding and suc- 

 cession, of the teeth: the base of the implanted tooth seldom 

 becomes consolidated, never contracted to a point, as in the 

 fangs of most mammalian teeth ; 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 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. 



The Dicynodonts 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 step towards the mammalian type of 



1 Transactions of the Cambridge Philosophical Society, vol. vii. part iii. 2 CLVIII. 



