upper jaw is trenchant, and seems to have been sheathed with horn.
The maxillary bone (21) is excavated by a wide and deep alveolus, with a circular area of half an inch, and lodges a long and strong, slightly curved, and sharp-pointed, canine tooth or tusk, which projects about two-thirds of its length from the open extremity of the .socket. The direction of the tusks is forwards, down wards, and very slightly inwards ; the two converging, as they descend along the outer side of the compressed symphysis of the lower jaw (c). The tusk is principally composed of a body of compact unvascular dentine. The base is excavated by a wide conical pulp.cavity (p) with the apex extend ing 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. From this central line the dentinal tubes radiate, with a gentle curve at the beginning, convex towards the point of the tusk, and then proceeding straight to the periphery of the tooth, but inclining towards the apex. They present parallel secondary curves, divide di chotomously twice or thrice near their begin ning, and send off numerous small lateral branches, chiefly from the side next the apex. At their primary curve the dentinal tubes are of an inch in diameter, and their .2 intervals are smooth of an inch across. The dentinal cells are most conspicuous near the periphery of the tooth, and vary in diameter from ,h-th to of an inch.
The enamel, at least at the middle of the tusk, is thinner than in the teeth of the cro codile. 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. There is no trace of teeth or their sockets in the lower jaw (25, 23); so much of the alveolar border as is exposed pre sents 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 ; and it is most probable, from the analogies of similarly-shaped jaws of existing Reptilia, that the fore part of both the upper and under jaws were sheathed with horn.
Until the discovery of the 1?hynelzosaurus*, this edentulous and horn-sheathed condition of the jaws was supposed to be peculiar to the Chelonian order among reptiles ; and it is not one of the least interesting features of the Dicynodonts of the African sandstones, that they should repeat a Chelonian character, hitherto peculiar, amongst Lacertians, to the above-cited remarkable extinct edentulous .genus of the new red sandstone of Shrop shire: but our interest rises almost to as tonishment, when, in a Saurian skull, we find, superadded to the horn-clad mandibles 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 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 consolidated, never contracted to a point, as in the fangs of the simple teeth of Mammalia, 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 dis placement. 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.
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 and much more important step towards the mammalian type of dentition, by maintaining the service able state of the tusk by virtue of constant renovation of the substance of one and the same matrix, accordingly to the principle manifested in the long-lived and ever-growing tusks of the Walrus, and the scalpriform in cisors of the Rodentia.