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Show 494 DR. R. BROOM ON THE STRUCTURE [Apr. 19, Though the known Theriodont genera differ greatly in their dentition, it will be observed that in the structure of the lower jaw and quadrate region they agree with each other very closely, and form by themselves a very distinct order. Much confusion has been caused in the past by there having been placed with the Theriodonts a large number of other forms which are not at all nearly related to them. I refer to such forms as sElurosaurus, and probably the majority of Owen's types with a similar dentition. While very little was known of these forms except that they had the dentition specialised as incisors, canines, and molars, it was but natural to place them with Galesaurus; but since the structure of the skull of these " primitive Theriodonts " has become known, it has been necessary to place them in a distinct order, which I have named Therocephalia (4). The Therocephalians differ from the Theriodonts in having, besides a large number of other distinctive features, a palate of the Rhynchocephalian type and a single occipital condyle. In the lower jaw the dentary, though possessed of a large coronoid process, is relatively much smaller than in the Theriodonts (see text-fig. 100, p. 496), while the angular is of large size, and the surangular and articular moderately Avell-developed. The Theriodonts are probably direct descendants of the Therocephalians, but the gap between the groups is very considerable. In Dicynodon and Udenodon the lower jaw is essentially similar to that in the Therocephalians, though the coronoid process is quite rudimentary. As the Dicynoclonts are probably, like the Theriodonts, also descended from the Therocephalians, the loss of the coronoid process is probably, as in the Monotremes, connected with the loss of the incisor teeth. The only other group to which the Theriodonts are closely related is the Mammalia. In the present paper I shall avoid any lengthy discussion of the question of the origin of mammals, but I wish to point out how nearly related the Theriodont jaw is to that of the mammal. The mammal differs from the reptile in having the jaw formed entirely of one bone-the dentary, and in the dentary articulating with the squamosal with apparently no quadrate. What has been the fate of the quadrate is a question which has received a number of very different answers. Owen regarded the tympanic bone as the mammalian equivalent of the quadrate, and this view has the support of Gadow (5) among others. Huxley and many others have argued in favour of one or other of the auditory ossicles being the mammalian quadrate, and the view that the incus is the quadrate appears to be the one chiefly supported by comparative anatomists at the present day. This latter view, though having the strong support of Kingsley (6) and Gaupp (7), has recently been very severely criticised by Gadow (8). Gadow not only shows that the mammalian auditory ossicles are together homologous with the columella and extra-columella of the Sauro-psida, but has shown how impossible it would be for an animal to |
