H'i Geological Society. 



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

 of the Stonesfield remains, and in part of their haA'ing 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 diiferent 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 0])ossum tliis 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 smaU 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 i-esembled 

 the jaw of an ordinary placental carnivorous or insectivorous mam- 

 mal ; but in both specimens of Thylacotherium the half-jaws of 

 Avhich exhibit their inner or mesial surfaces, this process presents 

 a fractured outline, evidently proving that when entire it must have 

 been produced inwards or mesiaUy, 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 spiirii 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 

 from the structure of the molares in any of the Phocs, to which these 

 very small Mammalia have been compared : and in reference to tliis 

 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 Phocidce, in 

 none of the species of which is the angle of the jaw so produced. The 

 Thylacotherium differs from the genusDidelphys 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 Cuvier urged as 

 evidence of the generic distinction of the Stonesfield mammiferous 



