The first important paper on _Thylacoleo_, which was a creature of the approximate size of a jaguar, whose remains are met with in the superficial formations of Australia, was one by Owen, published in the _Philosophical Transactions_ for 1859. From the general characters of the skull (which was at that time only known by fragments), and especially from the rudimentary condition of the hinder cheek-teeth and the enormous size of the secant replacing premolar, which bears a certain superficial resemblance to the carnassial tooth of the cats, its describer was led to the conclusion that _Thylacoleo_ was a marsupial carnivore, and “one of the fellest and most destructive of predatory beasts.” Probably Owen’s views at this time were, that the creature had its nearest living relatives in the members of the Australian family _Dasyuridæ_, such as the Tasmanian devil (_Sarcophilus ursinus_), and that it bore a relationship to the existing carnivorous marsupials somewhat similar to that presented by a lion to a dog. At this time there was no evidence to show whether the large teeth near the front of the jaw, the existence of which was indicated in the original specimen merely by its empty socket, was a canine or an incisor; and though Owen was inclined to regard it as the former, he admitted that it might be an incisor, in which event he recognised that the affinities of the animal would be more with the herbivorous, or diprotodont section of the marsupials, and more especially the phalangers, or so-called opossums of the colonists. This is clearly indicated by the following sentence appended by Sir Richard to his description:—“If, however, this be really the foremost tooth of the jaw, it would be one of a pair of terminal incisors according to the marsupial type exhibited by the _Macropodidæ_ (kangaroos) and _Phalangistidæ_ (phalangers).” In 1866, after receiving additional specimens from Australia, Owen was enabled to describe the greater part of the skull and the entire dentition of _Thylacoleo_. The large anterior teeth were clearly recognised to be incisors, which, in Owen’s opinion, “proved the _Thylacoleo_ to be the carnivorous modification of the more common and characteristic type of Australian marsupials, having the incisors of the lower jaw reduced to a pair of large, more or less procumbent and approximately conical teeth, or ‘tusks.’” Not only did the additional evidence serve to confirm Sir Richard in his view of the carnivorous propensities of _Thylacoleo_, but he considered that in this extinct form we have “the simplest and most effectual dental machinery for predatory life and carnivorous diet known in the mammalian class. It is the extreme modification, to this end, of the diprotodont type of marsupialia.”