Mr. Owen on the Thylacotherium. 57 



The teeth of Thylacotherium moreover differ from those of Didelphys not 

 only in number but also in size, being relatively smaller. The teeth of Myr- 

 mecobius, besides their approximation in number to those of Thylacotherium., 

 resemble them in their small relative size more than do those of Didelphys ; but 

 they are still smaller than in Thylacotherium, which, in this respect as well as in 

 the structure of the teeth, holds an intermediate position between Didelphys and 

 Myrmecobius. And it is interesting to find that the recent Mammal, which thus 

 approaches most nearly the extinct Thylacotherium, should exhibit in the structure 

 of its lower jaw, and of the articular cavity for the same, unequivocal evidence of 

 its belonging to the marsupial family. 



The Didelphys Prevostii being evidently, as Cuvier stated, a genus distinct from 

 Didelphys, properly so called, and as great inconvenience arises in applying to it 

 that name in the Linnsean sense, or as an equivalent to the term Marsupialis, — in 

 which sense alone it has ever been so applied by the naturalists of this country, — 

 M. Valenciennes has added to the value of his examination of the present fossil by 

 proposing for it a distinct generic appellation. It would have been more prudent, 

 perhaps, to have chosen one less descriptive than ' Thylacotherium,' since the affin- 

 ities of the fossil Insectivore to the marsupial order may be regarded as indicated 

 only with a certain degree of probability, and as requiring further evidence before 

 the desired demonstration can be attained. The determination of the particular 

 order of Mammals to which the animal represented by these ancient remains is 

 referrible, is a matter, however, of little moment, compared to the discovery of 

 the class of vertebrate animals to which it belonged. In reference to this point, 

 the evidence afforded by the two jaws, above described, decisively proves, in my 

 opinion, that they belonged to a true, warm-blooded, mammiferous species, refer- 

 rible also to the higher or unguiculate division of the class Mammalia, and to an 

 insectivorous genus ; with a probabiUty, according to the present known fossils, of 

 the marsupial character of such genus. 



At a subsequent Meeting I propose to bring forward my observations on the 

 more perfect fossil half-jaw, described by Mr. Broderip under the name of Di- 

 delphys Bucklandi, and of which the marsupial character is more strongly mani- 

 fested. 



As this, however, like the Thylacotherium, is generically distinct from the mo- 

 dern genus Didelphys, I shall describe it under the name of Phascolotherium Buck- 

 landi. 



The dental formula of the genus Thylacotherium, is, as regards the lower jaw, as 

 follows : 



Incisors j^^ ; canine i_i ; premolars ^ ; molars ^^ = ^. 



VOL. VI. SECOND SERIES. I 



