Genus PHALANGISTA.
The Phalangers, so called from the phalanges of the second and third digits of the hinder ex tremity being inclosed in a common sheath of integument, have the innermost digit modi fied to answer the purposes of a thumb ; and this hinder hand being associated in many of the species with a prehensile tail, they evidently, of all Frugivora, come nearest to the arboreal species of the preceding section. In a system framed on locomotive characters they would rank in the same section with the Opossums. We shall see, however, that they differ from those Entomophagous Marsupials in the con dition of the intestinal tube. Let us examine to what extent the dental characters deviate from those of the Opossums.
In the skull of a Phalangista Coo/tit, of which the dental formula is accurately given in fig. 86, there are both in the upper and lower jaws four true molars on each side, each beset with four three-sided pyramidal sharp pointed cusps ; thus these essential and most constant teeth correspond in number with those of the Opossum : but in the upper jaw they differ in the absence of the internal cusp, which gives a triangular figure to the grinding surface of the molars in the Opossum ; and the an terior single cusp is wanting in the true molars of the lower jaw. Anterior to the upper grinders in this Phalanger there are two pre molars of similar shape and proportions to those in the Opossum ; then a third premolar, too small to be of much functional importance, separated also, like the corresponding anterior premolar in the Opossum, by a short interval from those behind.
The canine tooth but slightly exceeds in size the above false molar, and consequently here occurs the first great difference between the Plialangers and Opossums ; it is, however, but a difference in degree of development ; and in the Ursine and other Phalangers, as well as in the Pelaurists, the corresponding tooth pre sents more of the proportions and form of a true canine.
The incisors, which we have seen to be most variable in number in the Carnivorous section, are here three instead of five on each side of the upper jaw, but their size, especially that of the first, compensates for their fewness.
In the lower jaw there is the same number of molars and functional premolars as in the Opossums ; the two very minute and function less molars, which form part of the same con tinuous series, represent the small premolar and canine of the upper jaw; and anterior to these there is one very small and one very large and procumbent incisor on each side. Now if this comparison be just and natural, the difference in the number of teeth between the Phalanger and the Opossum will resolve itself into the former being minus certain incisors in the up per and lower jaws : in the latter, the gmt development of the middle incisors seems to produce an atrophy of all the rest.
The interspace between the functionally de veloped incisors and molars in both jaws always contains in the Phalangers teeth of small size and little functional importance, and variable not only in their proportions but their number.
The constant teeth in the Phalangers are the 3 true molars, and the incisors.
4-4 1-1 The canines (c. fig. 86 and 87,) are constant in regard to their presence, but variable in size; they are always very small in the lower jaw.
With respect to the functional premolars 1-1 these are always in contact with the 1-1' molars, and their crowns reach to the same grinding level; sometimes a second premolar is similarly developed in the upper jaw, as in the Phal. Cookii, and as in the great flying Phalangers, (Petaurus Taguanoides, fig. 88) but it is commonly absent, or re placed by a very minute tooth, shaped like a canine ; so that in the upper jaw, between the posterior or functional premolar and the in cisors, we may find three teeth, of which the posterior is the largest, as in Phal. Cookii, or the smallest as in Phal. cevifi-ons ; or there may be only two teeth as in Phal. ursine and Phal. vulpine, and the species, whatever that may be, which M. Fr. Cuvier has selected as the type of the dentition of the Genus.