y. Scansoria.
Genus DIDELPHYS, (Opossums,fig. 85.) These Marsupials are now exclusively con fined to the American Continents, although the fossil remains of a small species attest their former existence in Europe contemporaneously with the Palaothere, Anoplothere, and other extinct Pachyderms, whose fossil remains cha racterize the Eocene strata of the Paris Basin. The dental formula of the Genus Didelphys is,— Incisors, 5-5 ; canines, 1-1 '• prw 4-4 - 1-1molars, • molars, 4-4 .= 50.
3-3' 4-4 The Opossums resemble in their dentition the Bandicoots more than the Dasyures : but they closely resemble the latter in the tuberculous structure of the molars. The two middle in cisors of the upper jaw are more produced than the others, from which they are also separated by a short interspace. The canines are well de veloped; the upper being always stronger than the lower. The false molars are simply conical, but are more compressed than in the Carnivo rous Marsupials. The posterior false molar is the largest in the upper jaw ; the middle one is the largest in the lower jaw ; the anterior one is the smallest in both jaws. The true molars are beset with sharp cusps which wear down into tubercles as the animal advances in age. The crowns of the upper molars present a triangular horizontal section : the base of the triangle is turned forward in the posterior mo lar ; and obliquely inwards and outwards in the rest. In the lower jaw the true molars are narrower and of more equal size than in the upper jaw : there are five tubercles on each, four placed in two transverse pairs, the anterior being the highest, and a fifth forming the anterior and internal angle of the tooth : the anterior and external angle seems as if it were vertically cut off.
The smaller species of Didelphis, which are the most numerous, fulfil in South America the office of the insectivorous Shrews of the old Continent. Their external resemblance is so close that some have been described as spe cies of Sores, but no true representative of this placental genus has hitherto been disco vered in South America. The larger Opossums
resemble in their habits, as in their dentition, the Carnivorous Dasyures, and prey upon the smaller quadrupeds and birds, but they have a more omnivorous diet, feeding on reptiles and insects and even fruit. One large species, ( Did. cancrivora) prowls about the sea shore and lives, as its name implies, on crabs and other crustaceous animals. Another spe cies, the Yapock, frequents the fresh waters, and preys almost exclusively on fish. It has all the habits of an Otter ; and, in consequence of the modifications of its feet, forms the type of the sub-genus Cheirenectes, Ill. Besides being web-footed the anterior extremities pre sent an unusual development of the pisiform bone, which supports a fold of the skin, like a sixth digit; it has indeed been described, as such, by M. Temminck : this process has not, of course, any nail. The dentition of the Yapock resembles that of the ordinary Didelphis. All the Opossums have the inner digit of the hind foot converted by its position and development.
into a thumb, but without a claw. The hinder hand is associated in almost all the species with a scaly prehensile tail.
In some of the smaller Opossums the sub abdominal tegumentary folds are rudimental, or merely serve to conceal the nipples, and are not developed into a pouch : the young in these species adhere to the mother by entwining their little prehensile tails around her's ; and they cling to the fur of the back, hence the term dor•igcra applied to one of these Opos sums.* Tribe HI. CARPOPHAGA. Stomach simple ; ccecum very long.
In this family the teeth, especially those at the anterior part of the mouth, present consider able deviations from the previously described formula: ; the chief of which is a predomi nating size of the two anterior incisors, both in the upper and lower jaws. hitherto we have seen that the dentition in every marsupial genus has participated more or less in a carni vorous character ; henceforth it will manifest a tendency to the Rodent type.