There is no toothless genus among the true Marsupials, unless the Monotremes which re present the Edentate order of the Placental Mammalia be regarded as modified Marsupials. Molar and incisor teeth are present in both jaws in every true Marsupial species ; the ca nines are but feebly represented in many, as the Phalangers, Petaurists, &c. are wanting in the lower jaw in the I'otoroos and Koala, and in both jaws in the Kangaroos and Wombat. The grinders, on the other hand, present their most complicated structure in these last cited herbi vorous genera.
The Dasyures and Thylacine offer the carni vorous type of the dental system, but differ from the corresponding group of the Placental Mammalia in having the molars of a more uniform and simple structure, and the incisors in greater number ; which number, however, is different in the different Marsupial carnivorous genera, as is expressed in the dental formulni already given.
The canines are as formidable for their size, shape, and strength in the Thylacine and Ur sine Dasyure, as in the Dog or Cat, and in a fossil species of the latter genus ( Dasyurns laniarius),* which co-existed, in ancient Aus tralia, with herbivorous Marsupials of greater size than now inhabit that continent, these teeth were as large as in the Leopard. In the Thylacine the points of the lower ca nines are received in hollows of the intermaxil lary palatal plate when the mouth is closed, and do not project, as in the carnivorous pla centals, beyond the margins of the intermaxil lades.
In some of the smaller species of the carni vorous group, as the Phascogales, the canines lose their great relative size, and the molar teeth present a surface more cuspidated than sectorial : there is also an increased number of teeth, and as a consequence of their more equa ble development they have fewer and shorter interspaces. Thus the Phascogale penicillata has, as Mr. Bunter observed, " a mouth full of teeth," and these are adapted for the capture and mastication of insects and other small and low organized animals.
In the Opossums the canines still exhibit a superior development in both jaws adapted for the destruction of living prey, but the molars have a conformation different from that which characterises the true flesh-feeders, and they consequently subsist on a mixed diet or lower organized animals ; some, as the web-footed Cheironcacs, betake themselves to the water, and prey, like the otter, on fish ; others prowl about the sea-shore and subsist on crustacca, as the Didc1phys cuncrivora.
The Pen:metes are for the most part insecti vorous ; the incisors are always very small, the molars generally multicuspidate; some species, as Per. nasula, have the canines not more de veloped than the premolars, which they closely resemble ; but in others, as the Pcr. lagolis, they are proportionally as large as in the Opossums, and the inferior ones are concealed in the same position when the mouth is closed, as in the Thylacine. But the Per. lagotis, in
stead of exhibiting a corresponding approach in the structure of the molars to a carnivorous diet, have these unerring indicators of the na ture of the food terminated by a broad oblique flattened surface, adapted to the trituration of farinaceous vegetable roots, the destruction of which is confidently attributed to this species of Bandicoot by the colonists of Swan River. The interesting genus Alyrmecobius offers, in the small size and scattered distribu tion of its teeth, the nearest approach among the Marsupials to the edentate group of the Placental Mammalia ; the multicuspidate struc ture of the molar teeth, and their small size, indicates that The Myrmecobius feeds chiefly on the weaker insects which are implied by its generic name. It is important to notice that in many of the Marsupials there is an inconstancy in the number 'of teeth in species of the same genus, and sometimes even in individuals in the same species; this at least appears to be the case in the Illynnecolnus, in which, of three specimens examined, identical in all other respects, one had forty-eight, the other fifty-two, and the third fifty teeth. We have already pointed out the variety which ob tains in the spurious and true molars of the Phalangers and Petaurists. The prominent feature in the change from the carnivorous to the herbivorous type of dentition is the inordi nate development of the two middle incisors of the lower jaw, at the expense, as it would seem, oldie posterior ones. In the Phalangista Cookii, e. g. six incisors are always present in the lower jaw, but only the first two have any functional character : the canine tooth again offers neither a form nor size by which it can be distinguished from the spurious molars; in the other Mar supials with the same characteristic modifica tion of the binder feet, including the Petaurists with the Phalangers, the small posterior incisors are wanting wholly or in part ; the canines seem also to be lost, and the spurious molars are fewer, but variable in number. This incon stancy is not to be wondered at in teeth which have too simple a form and too insignificant a size to exercise any influence on the habits and economy of the species; and it would seem to he lost labour in the zoologist to attempt to found generic distinctions, and invent new names for the species in which these insignifi cant varieties are presented. The six incisors of the upper jaw and the two anterior ones in the lower jaw, with the true molars in both jaws, present a constancy of character and a functional importance in their development in all the Phalangcrs and Petaurists. These teeth are adapted, the first for cropping the foliage of the gum-trees ( Eucalypti) and similar trees, and the others for bruising and masticating the same. The grinding surface of the molars ge nerally presents four blunt tubercles.