8 
vertical plate of bone, a more or less flattened triangular surface or 
plate of bone extended between the external ridge and the internal 
process or inflected angle. In the Opossum this process is triangu- 
lar and trihedral, and directed inwards with the point slightly curved 
upwards and extended backwards, in which direction it is more pro- 
duced in the small than im the large species of Didelphys. 
Now, if the process from the angle of the jaw in the Stonesfield 
fossil had been simply continued backwards, it would have resembled 
the jaw of an ordinary placental carnivorous or insectivorous mam- 
mal; but in both specimens of Thylacotherium the half-jaws of 
which exhibit their inner or mesial surfaces, this process presents 
a fractured outline, evidently proving that when entire it must have 
been produced inwards or mesially, as in the Opossum. 
Mr. Owen then described in great detail the structure of the teeth, 
and showed, in reply to M. de Blainville’s second objection, that they 
are not confluent with the jaw, but are separated from it at their 
base by a layer of matter of a distinct colour from the teeth or the 
jaw, but evidently of the same nature as the matrix ; and secondly, 
that the teeth cannot be considered as presenting an uniform com- 
pressed tricuspid structure, and being all of one kind, as M. de 
Blainville states, but must be divided into two series as regards their 
composition. Five if not six of the posterior teeth are quinque-cus- 
pidate and are molares veri; some of the molares spurii are tricuspid 
and some bicuspid, asin the Opossums. An interesting result of this 
examination is the observation that the five cusps of the tuberculate 
molares are not arranged, as had been supposed, in the same line, 
but in two pairs placed transversely to the axis of the jaw, with the 
fifth cusp anterior, exactly as in the Didelphys, and totally different 
from the structure of the molares in any of the Phocz, to which these 
very small Mammalia have been compared: and in reference to this 
comparison, Mr. Owen again calls attention to the value of the cha- 
racter of the process continued from the angle of the jaw, in the 
fossils, as strongly contradistinguishing them from the Phocidz, in 
none of the species of which is the angle of the jaw so produced. The 
Thylacotherium differs from the genus Didelphys in the greater num- 
ber of its molars, and from every ferine quadruped known at the time 
when Cuvier formed his opinion respecting the nature of the fossil. 
This difference in the number of the molar teeth, which Cuvierurgedas 
evidence of the generic distinction of the Stonesfield mammifercus 
fossils, has since been regarded as one of the proofs of their Saurian 
nature; but the exceptions by excess to the number seven, assigned 
by M. de Biainville to the molar teeth in each ramus of the lower 
jaw of the insectivorous Mammalia, are well established, and have 
been long known. ‘The insectivorous Chrysochlore, in the order 
Fere, has eight molars in each ramus of the lower jaw; the insec- 
tivorous Armadillos have not fewer ; and in one subgenus (Priodon) 
there are more than twenty molar teeth on each side of the lower 
jaw. The dental formule of the carnivorous Cetacea, again, de- 
monstrate the fallacy of the argument against the mammiferous cha- 
racter of the Thylacotherium founded upon the number of its molar 
