In Petaurussciurcus and Petaurusflaviventer the dentition more nearly resembles that of Phalangista vulpine. In the upper jaw the functional molar series consists of five teeth on each side ; the four hinder ones being, as in Pet. 2'aguanoides, true tuberculate molars, but diminishing more rapidly in size as they are placed further back in the jaw; the hinder tooth has three tubercles, the rest four ; the apices seem to be naturally blunter than in Pet. Taguanoides.
Between the functional przemolar and the incisors there are three teeth, of which the is relatively smaller, and the first of a more triangular form than in Pet. Taguanoidcs. The space between the tuberculate molars and the procumbent incisor is occupied by four small teeth, of which the one immediately anterior to the molars is large, compressed, pointed, and has two roots; the remaining three are rudi mentary and have a single fang; the anterior of these corresponds to the one regarded as canine in the upper jaw.
Among the species exhibiting this dental formula, viz.
Incisors 3-3 1-1 ; canines ; premo 1-1 1-1 44 lairs 3-3 ; molars = 40, 3-3 4-4 are Pet. sciureus, Pet. flaviventer, and Pct.
seacrurus.
The Pigmy Petaurist differs from the pre ceding and larger species, in having the hairs of the tail distichous, or arranged in two regular lateral series like the barbs of a feather, and in having the spurious molars large and sharp-pointed ; and the true molars bristled each with four acute cusps. This tendency in the dentition to the insectivorous character, with the modification of the tail, induced M. Desmarest to separate the Pigmy Petaurist from the rest of the species, and constitute a new sub-genus for its reception under the name of Acrobates.* To Mr. Waterhouse, however, is due the credit of having first pointed out that the Pigmy Petaurist had but three true molars on each side of each jaw instead of four. There seems, there fore, to he better reason for accepting this sub generic section, although we evidently perceive a transition to this condition in the small size of the hinder or fourth molars in the Sciurine Petaurist and its congeners.
The description of the dentition of the Pigmy Petaurist in the Itegne Animal, besides being defective in this remarkable particular, is not quite exact in other respects. In four adult specimens, two of which were males, and two females with young in the pouch, I find the fol lowing dental formula to be constant (fig. 90).
representative of the canine is relatively much larger than in the Pet. Taguanoides; the first false molar is also larger and has two roots; the second, which is functional in Pet. Taguanoides, is here very small. The canine is more deve loped ; the first incisor is also relatively larger and more produced. In the lower jaw the functional series of grinders consists of the four true tuberculate molars only, of which the last The three quadricuspidatcgrinders of the upper jaw are preceded by three large premolars, each of which has two fangs, and a compressed triangular sharp-pointed crown, slightly but pro Axpo;, stimmus, Ramer, gradior, as frequentin.; the summits of trees.
gressively increasing in length as they are placed forward. An interspace occurs between these and the canine, which is long, slender, sharp-pointed, and recurved. The first incisor is longer than the two behind, but is much shorter than the canine. In the lower jaw the
true molars are preceded by two functional false ones, similar in size and shape to the three above the anterior false molar, and the canine are represented by minute rudimental simple teeth; the single incisor is long and procumbent as in the other Petaurists.
With these differences of dentition approach ing more or less to one or other of the modifi cations of the dentition in the group of Phalan gers, the Petaurists may nevertheless be readily discriminated from those Phalangers which they most resemble for example, the Petaurus Taguanoides may be distinguished from the Phalangista Cookii by the greater relative length in the latter of the nasal and maxillary portion of the skull; while in most of the other species of Petaurus, the facial part of the skull is relatively shorter than in the Pet. l'agua noides.
Genus PIIASCOLARCTUS.
The absence of anomalous or functionless premolars and of inferior canines appears to be constant in the only known species of this genus. The dental formula in three examples of this species ( Phasc.fitscus, Desm.) is Incisors canines premo 1-1. 0-0 lars ; molars 4-4 30. ( Fig. 95.) 1-1 4-4 The true molars are larger in proportion than in the Phalangers ; each is beset with four three-sided pyramids, the cusps of which wear down in age, the outer series in the upper teeth being the first to give way ; those of the lower jaw are narrower than those of the upper. The spurious molars are compressed and terminate in a cutting edge; in those of the upper jaw there is a small parallel ridge along the inner side of the base. The canines slightly exceed in size the posterior incisors they terminate in an oblique cutting edge rather than a point ; their fang is closed at the extremity : they are situated as in the Phalangers close to the inter maxillary suture. The lateral incisors of the upper jaw are small and obtuse; the two ante rior or middle incisors are twice as long, broad, and thick as the posterior incisors; they are conical, slightly curved, sub-compressed, be velled off obliquely to an anterior cutting edge, but differing essentially from the denies scat prarii of the Rodentia in being closed at the extremity of the fang. The two incisors of the lower jaw resemble those of the upper, but are larger and more compressed ; they are also formed by a temporary pulp, and its absorption is accompanied by a closure of the aperture of the pulp cavity, as in the upper incisors. The Koala, therefore, in regard to the number, kind, and conformation of its teeth, closely resembles the Phalangers, with which it also agrees in its long ccecum and the general conformation of its digestive organs. It has also the extremities similarly organised for prehension qtach is ter minated by five digits; the hind feet are provided with a large thumb, and have the two contiguous digits enveloped in the same tegumentary fold ; the anterior digits are divided into two groups the thumb and index being opposed to the other three fingers. We have already noticed a structure approaching to this in some of the small Phalangers. The Koala, however, differs from the Phalangers and Petaurists in the ex treme shortness of its tail, and in its more com pact and heavy general form. It is only known to feed on the buds and leaves of the trees in which it habitually resides.