The Koala resembles in its dentition and in its diet and arboreal life the Phalangers and Petaurists. In the lower jaw the absence of teeth between the two large procumbent incisors and the false molars is constant ; in the upper jaw, however, the lateral or posterior incisors begin to exhibit the same diminution of size as the corresponding teeth in the lower jaw of some of the Phalangers, while the two anterior upper incisors present a proportional increase ; the canines correspond in feebleness and form with the small incisors. In the Wombat, the defective development to which the teeth be tween the incisors and molars are subject in the Marsupial tribe, has reduced the dental for mula of both jaws to the rodent type ; but the shape, comparative shortness, and procumbent position of the two large inferior incisors, and the three-sided figure of the opposing pair above, bespeak the marsupial character of which this genus offers so extreme a modification. The grinders, however, agree with those of the herbivorous rodents in the absence of Fangs, arising from the uninterrupted growth and ossi fication of their formative pulps ; they offer also in both jaws an extreme degree of the cur vature which characterises the molars of some herbivorous Rodents, as the Guinea-pig, and the great extinct gliriform Pachyderm, called Toxodon; but their chief distinctive peculi arity is the marsupial excess of number already mentioned.
With respect to the modifications of the teeth of the herbivorous Marsupials, it need only here be observed that the grinding surface of the true molars in the Kangaroos most re sembles that which characterizes the same teeth in the Tapir, Dinothere, and Manatee.
No Marsupial possesses teeth composed of an intermixture of layers of dentine, enamel and cement throughout the crown, but the ex ternal layer of coronal cement is very conspi cuous in transverse sections of the teeth of the Marsupials, viewed with the microscope, and forms a thick layer on the outside of the crowns of the teeth in the genera iliac, opus, Phascol arctus, and Phascolomys.
The modifications of the tongue and soft palate have already been noticed. In two species of Marsupials I have detected cheek pouches. In the Koala they are wide and shallow, situated one on each side of the upper lip ; the orifice is opposite the first superior premolar, and leads forwards above a horizontal fold of the mucous membrane which attaches the upper lip to the side of the intermaxillary bone, separating this part of the cheek-pouch from the mouth. In the Perameles lagotis there are also two small fossm, one on the in side of each cheek, about four lines in diameter, and lined by a very distinct white epithelium.
The fauces are wide in the Zoophagous, but narrow in the Entomophagous and Phytopha gous Marsupials.
Alimentary canal.—The esophagus in passing through the chest recedes from the spine as it approaches the diaphragm, and is loosely con nected with the bodies of the dorsal verte bra; by a broad duplicature of the serous membrane of the posterior mediastinum. In the Phalangers the oesophagus terminates in the stomach almost as soon as it has pierced the diaphragm ; in the Opossums it is continued some way into the abdomen ; in the Didelphys Virginiana, for example, for the extent of an inch and a half; in Did. bra.chyura, for half an inch. In the Kangaroos the abdominal portion of the oesophagus is of still greater extent ; 1 have observed it five inches long in a male Macropus major.
The inner surface of the oesophagus is gene rally smooth, or disposed in fine longitudinal plaits ; but in the Virginian Opossum the ter minal part of the (esophagus presents many transverse folds of the lining membrane analo gous to, but relatively larger than those in the Lion and other Felines. I have not met with a like structure in the Plialangers nor in any other genus of Marsupials ; what is more re markable is that the transverse (esophageal rug are not developed in the carnivorous Dasijures or Phascogales, where analogy would lead one to expect them, rather than in the insectivorous Opossums.
The stomach presents three leading modifica tions of structure in the Marsupialia; it is either simple, as in the Zoophagous, Entomophagous, and Carpophagous tribes; or is provided with a cardiac glandular apparatus, as in the Koala and Wombat; or is complicated by sacculi, as in the Poephagans.
It might have been expected that the stomach would have exhibited some modifications in the development of the left or cardiac extremity corresponding with the differences of food and dentition observable in the large proportion of the Marsupial order, in which this viscus pre sents its simple condition ; but this is not the case : the form of the stomach is essentially the same in the carnivorous Dasyure, the insecti vorous Bandicoot, and the leaf-eating Phalan gers. It presents a full, round, ovate, or sub-triangular figure, with the right extremity projecting beyond and below the pylorus ; the longitudinal diameter seldom exceeds the ver tical or transverse by more than one-third ; often, as in Phascogale and Dasyurus uiverrinus, by only one-fourth of its own extent ; and the (esophagus enters at the middle of the lesser curvature, or sometimes nearer the pylorus, but always leaves a large hemispherical cul-de-sac on the left side. Daubenton has given illustra tions of this characteristic form of the stomach in different species of Didelphys ; it is here figured as it exists in the Phascogale (jig. 122). The stomach is relatively much more capacious in the carnivorous Marsupials than in the car nivorous Placeutals. Some slight modifica tions occur in the disposition of the lining membrane ; thus in the Phascogale I observed a series of very thick rugm radiating from the middle of the upper part of the caeca] end of the stomach, some of which were continued along the lesser curvature to the pylorus. Dr. Grant found, in the Peramcles nasuta, that " the Beaver, but in this animal the excretory orifices of the gland are arranged in three longitudinal rows, while in the Wombat and Koala they are scattered irregularly ; in the Wombat they are about thirty in number, and the bottoms of the larger depressions are subdivided into smaller cells. In the partially contracted state the inner membrane of the stomach of the Wombat, as represented in the figure, is dis posed in pretty regular longitudinal rugle, which gradually subside towards the pylorus ; but when the stomach is distended these folds disappear, and the left extremity presents a full globular form. In the Wombat dissected by me the (esophagus terminated nearer the py lorus than is represented in the figure here given from the Comparative Anatomy of Sir Everard Home.