The eight incisors of the upper jaw are of the same length and simple structure, and are arranged in a regular semicircle without any median interval. The six incisors of the lower jaw are similarly arranged, but have thicker crowns than the upper ones. The canines present the same or even a greater relative de velopment than in the Thylacine : in an extinct species of Dasyurus* they had the same form and relative proportions as in the Leopard. The spurious molars have a pointed com pressed triangular crown with a rudimental tu bercle at the anterior and posterior part of its base. The grinding surface of the true molars in the upper jaw is triangular; the first presents four sharp cusps, the second and third each five, the fourth, which is the smallest, only three. In the lower jaw the last molar is nearly of equal size with the penultimate one, and is bristled with four cusps, the external one being the longest : the second and third molars have five cusps, three on the inner and two on the outer side; the first molar has four cusps : these are all sharply pointed in the young animal, in which the posterior tubercle of the posterior molar in the lower jaw is divided into two small cusps.
The carnivorous character of the above den tition is most strongly marked in the Ursine Dasyure or Devil of the Tasmanian Colonists, the largest existing species of the genus, and a most pestilent animal in the poultry-yard or larder.
Genus PHASCOGALE. ( Fig. 82.) In the present dental formula may be dis cerned a step in the transition from the Da syures to the Opossums, not only in the in creased number of spurious molars, but also in the shape and proportions of the incisors. In the upper jaw the two middle incisors are longer than the rest, and separated from them by a brief interval ; they are more curved and project more forward. The three lateral in cisors diminish in size to the outermost. The middle incisors of the lower jaw also exceed the lateral ones in size, and project beyond them but not in the same degree, nor are they separated from them by an interval, as in the upper jaw. The canines are relatively smaller than in the Dasyures. The spurious molars present a similar form, but the third in the lower jaw is smaller and simpler than the two preceding ones. The true molars resemble those of the Dasyures.
The general character of the dentition of these small predatory Marsupials approximates to the insectivorous type, as exemplified in the Shrew, Hedgehog, &c. among the placental Mammalia, and corresponds with the food and habits of the species which thus lead from the Sarcophagous to the Entomophagous tribes.
The interval is further diminished by a lost marsupial genus which forms one of the ancient Mammalia that have rendered the oolitic formations at Stonesfield so celebrated. This genus, which I have called Phascolothe riurn , presents the same numerical dental formula as in Phascogale, viz.
? ?—?Incisors — canines ; 3-3 or 4-4 ; ?•-•-? premolars 3... molarsBut the incisors and canines are separated by vacant interspaces, and occupy a large pro portional space in the dental series : the true molars resemble those of Thylacine.
Tribe II. ENTOMOP II AGA .
This is the most extensive and varied of the primary groups of the Marsupial order. In the system of Cuvier, the species of this tribe are united with those of the preceding to form a single group characterized by the presence of long canines and small incisors in both jaws : but in most of the Entomophagous genera of the present classification, the canines present a marked inferiority of development, and the species are consequently unable to cope with animals of their own size and grade of organ ization, but prey, for the most part, upon the smaller and weaker classes of invertebrate animals. Their intestinal canal is complicated by a moderately long and large ccecum ; and while, in the Sarcophaga, the feet are con structed upon the plan of those of the ordinary placental Digitigrades, they offer in the pre sent tribe a variety of well-marked modi fications, according to which the species may be arranged into gressorial, saltatory, and scan sorial groups.
a. G ressoria.
Genus M YRM ECOBI US.
The only known existing representative of this family is the animal described by Mr. Waterhouse, which constitutes the type of his genus lllyrmecobius, and of which the following is the remarkable dental formula. ( Fig. 83.) general external form and bushy tail, it offers an especial approximation to the genus Phus cogale.
S. Saltatoria.
Genus PERA M EL ES. (Bandicoots,fig. 84.) From this formula it will be seen that the number of molars, eighteen in both jaws, ex ceeds that of any other known existing mar supial, and nearly approaches the peculiar dental formula of the extinct Thylaeotherium,* and that which characterizes some of the ex isting Armadillos. The resemblance to the genus Dasypus is further carried out in the small size of the molar teeth, their separation from each other by slight interspaces, and their implantation in sockets, which are not formed upon a well-developed alveolar ridge or process. The molars, however, present a distinct multi cuspidate structure, and both the true and false ones possess two separate fangs, as in other Marsupials. The inferior molars are directed obliquely inwards, and the whole dental series describes a slight sigmoid curve, (fig. 97.) The false molars present the usual compressed triangular form with the apex slightly recurved; and the base more or less obscurely notched before and behind. The canines are very little longer than the false molars; the incisors are minute, slightly com pressed and pointed; they are separated from each other and the canines by wide intervals.