_Plagiaulax_, of the Dorsetshire Purbeck (Figs. 24, 25), is at once distinguished from _Tritylodon_ by its secant premolars, which, as already mentioned, recall those of some of the _Macropodidæ_, although readily distinguished by the convexity of the cutting edge and their oblique grooving. This remarkable and highly specialised type has been the occasion of one of the most interesting discussions on the inferences which may be drawn as to the affinities and habits of an otherwise unknown animal from the structure of a small portion of its organisation which occurs in the annals of natural history—a discussion carried on with great ability, ingenuity, and wealth of illustration on both sides. Dr. Falconer maintained that it was more nearly allied to the Rat-Kangaroo (_Potorous_ or _Hypsiprymnus_) than to any other existing form, and that, as it is known that these animals feed upon grass and roots, “it may be inferred of _Plagiaulax_ that the species were herbivorous or frugivorous. I can see nothing in the character of their teeth,” he adds, “to indicate that they were either insectivorous or omnivorous.” Sir R. Owen, on the other hand, from the same materials came to the conclusion that “the physiological deductions from the above-described characteristics of the lower jaw and teeth of _Plagiaulax_ are that it was a carnivorous Marsupial. It probably found its prey in the contemporary small insectivorous mammals and Lizards, supposing no herbivorous form like _Stereognathus_ to have co-existed during the Upper Oolitic period.” It is impossible here to give at any length the arguments by which these opposing views are respectively supported, but it may be indicated that the first-mentioned is strongly countenanced by the consideration of the following facts: (1) all existing Marsupials may be divided, so far as their dentition is concerned, into two groups—(_a_) those which have a pair of large more or less procumbent incisors close to the symphysis of the lower jaw, and rudimentary or no canines (diprotodont dentition), and (_b_) those which have numerous small incisors and large pointed canines (polyprotodont dentition); (2) the vast majority of the former group are purely vegetable feeders, and almost all of the latter are carnivorous or insectivorous; and (3) _Plagiaulax_, so far as its structure is known, shows an analogy with the former group; and, as we have no sure basis for inferences as to the habits of an unknown animal, but the knowledge of the habits of such as are known, we have no grounds for supposing that its habits differed from those forms having an analogous type of dental structure.[31]