FOSSIL JAWS FROM STONESFIELD. 203 



the true articular condyle, has been mistaken for " une sorte d'echancrure 

 articulaire, un peu comme dans les poissons." 



" The specimen of the half-jaw of the thylacothere examined by M. Va- 

 lenciennes, like that which was transmitted to Cuvier, presents the inner 

 surface to the observer, and exhibits both the orifice of the dental canal and 

 the symphysis in a perfect state. The foramen in the fossil is situated re- 

 latively more forward than in the recent opossum and dasyiire, or in the 

 placental Insectivora, but has the same place as in the marsupial genus 

 Hypsiprymnus. The symphysis is long and narrow, and is continued for- 

 ward in the same line with the gently convex inferior margin of the jaw, 

 which thus tapers gradually to a pointed anterior extremity, precisely as in 

 the marsupial Insectivora. In the relative length of the symphysis, its form 

 and position, the jaw of the Thylacotherium 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 respecting them, but which, if co-ex- 

 istent 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 inferior 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 opos- 

 sum this process is triangular and trihedral, and directed inwards with the 

 point slightly curved upwards and extended backwards, in which direction 

 it is more produced 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 mammal ; 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 consider- 

 ed as presenting an uniform, compressed, 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-cuspidate, 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 

 molars 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 

 molars in any of the Phocce, to which these very small Mammalia have been 

 compared : and in reference to this comparison, Mr. Owen again calls at- 

 tention to the value of the character of the process continued from the an- 

 gle 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 produ- 

 ced. The Thylacotherium differs from the genus Didelphys in the greater 



