[Illustration: From Flower, _Quart. Jour. Geol. Soc._
FIG. 2.--Front View of Skull of the Tasmanian Devil (_Sarcophilus ursinus_) to exhibit polyprotodont type of dentition.]
The first family is that of the true or American opossums--_Didelphyidae_, in which there are five pairs of upper incisors, while the feet are of the presumed primitive arboreal type, the hind foot having the four outer toes sub-equal and separate, with the first opposable to them all. With the exception of the water-opossum, forming the genus _Chironectes_, all the living members of the family may be included in the genus _Didelphys_. The latter may, however, be split up into several sub-generic groups, such as _Metachirus_, _Philander_, _Marmosa_ (_Micoureus_ or _Grymaeomys_), _Peramys_, _Dromiciops_, &c. The small South American forms included in _Marmosa_, which lack the pouch, and have numerous teats, and molar teeth of a primitive type, are doubtless the most generalized representatives of the group (see OPOSSUM; and WATER-OPOSSUM).
Nearly allied is the Australian family _Dasyuridae_, characterized by the presence of only four pairs of upper incisors, the generally small and rudimentary condition of the first hind toe, which can but seldom be opposed to the rest, and the absence of prehensile power in the tail; the pouch being either present or absent, and the fore feet always five-toed. The stomach is simple, and there is no caecum to the intestine, although this is present in the opossums.
The largest representative of the family is the Tasmanian wolf, or thylacine, alone representing the genus _Thylacinus_, in which the dentition numbers i. 4/3, c. 1/1, p. 4/4, m. 3/3 = 46; with the incisors small and vertical, the outer one in the upper jaw being larger than the others. Summits of the lower incisors, before they are worn, with a deep transverse groove, dividing it into an anterior and a posterior cusp. Canines long, strong and conical. Premolars with compressed crowns, increasing in size from before backwards. Molars in general characters resembling those of _Sarcophilus_, but of more simple form, the cusps being less distinct and not so sharply pointed. Deciduous molar very small, and shed before the animal leaves the mother's pouch. General form dog-like, with the head elongated, the muzzle pointed, and the ears moderate, erect and triangular. Fur short and closely applied to the skin. Tail of moderate length, thick at the base and tapering towards the apex, clothed with short hair. First hind toe (including the metacarpal bone) absent. Vertebrae: C. 7, D. 13, L. 6, S. 2, Ca. 23. Marsupial bones unossified. The gradual passage of the thick root of the tail into the body is a character common to the Tasmanian wolf and the aard-vark, and may be directly inherited from reptilian ancestors (see THYLACINE).
