OF THE MARSUPIALIA. 321 
shape, and these are separated by intervals, as in Myrmecobius. The marsupium opens 
downwards in the Cheropus, as in the true Bandicoots. The species described has no 
tail. The genus would seem by its dentition to rank between Myrmecobius and Pera- 
meies. ts digital characters are anomalous and unique among the Marsupialia. 
y. ScANsoria. 
Genus Didelphys (Opossums). 
These Marsupials are now exclusively confined to the American Continents, although 
the fossil remains of a small species attest the former existence of the genus Didelphys 
in Europe contemporaneously with the Paleothere, Anoplothere, and other extinct Pa- 
chyderms whose fossil remains characterize the Eocene strata of the Paris Basin. The 
dental formula of the genus Didelphys is, 
‘4 55 ‘ jh 4] 38—3 4—4 
—3 Sse wae ——~: = 50. 
Incisors 4—4' Canines ;— ; premolars aaa molars . 50 
The Opossums resemble in their dentition the Bandicoots more than the Dasyures, 
except in the structure of the molares. 
The two middle incisors of the upper jaw are more produced than the others, from 
which they are separated by a short interspace. The canines are well developed, the 
upper being always stronger than the lower. The false molars are simply conical, but 
more compressed than in the Zoophagous Marsupials. The posterior false molar is the 
largest in the upper jaw; the middle one is the largest of the three in the lower ; the 
anterior one is the smallest in both jaws: in the upper jaw it is separated from the 
middle false molar by a short interspace, and the same character occurs in the lower 
jaw in Didelphys Virginiana ; but in the species Cancrivora, Crassicaudata, Nudicaudata, 
Opossum, and others, this diastema is very slightly marked, or is wanting. A small 
accessory posterior cusp, and sometimes, though rarely, a still smaller anterior cusp, are 
added to the base of the principal compressed cone, which forms the crown of the spu- 
rious molars. The sharp cusps of the true molars wear down into tubercles as the 
animal advances in age. 
The true molars in the upper jaw present a triangular horizontal section: in the 
posterior molar the base of the triangle is directed forwards ; in the rest one side of the 
triangle looks outwards ; another forwards, at right angles to the preceding ; and the 
third obliquely inwards and backwards. The triturating surface of the crown in re- 
cently formed teeth is bristled with several sharp tubercles, of which the largest rises 
from the middle of the posterior side, and others at each of the angles of the crown: 
there are smaller tubercles intermediate to these. The posterior molar is smaller than 
the rest, and has fewer tubercles. In the lower jaw the true molars are narrower, of 
more equal size than in the upper: there are five tubercles on each, four in two trans- 
verse 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. 
VOL. 11.— PART Iv. 20 
