64 Geological Society. 



precisely as in the jaws of the Marsupial Insectivora. In the relative 

 length of the symphysis, its form and position, the jaw of the Thy- 

 lacotherium precisely corresponds with that of the Didelphys. 



In addition, however, to these proofs of the mammiferous nature 

 of the Stonesfield remains, and in part of their having belonged to 

 Marsupialia, Mr. Owen stated that the jaws exhibit a character 

 hitherto unnoticed by the able anatomists who have written respect- 

 ing them, but which, if co-existent with a convex condyle, would 

 serve to prove the marsupial nature of a fossil, though all the teeth 

 were wanting. 



In recent marsupials the angle of the jaw is elongated and bent 

 inwards in the form of a process, varying in shape and development 

 in different genera. In looking, therefore, directly upon the infe- 

 rior margin of the marsupial jaw, we see in place of the edge of a 

 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 in 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, as in 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 



