THE FOSSIL JAWS FROM STONESFIELD. 9 



angle of the jaw is just as well adapted in the Rodentia for 

 the action of their teeth. 



Thus the form of this process, and that of the teeth and of 

 the condyle, are always combined in such a manner that the 

 study of these parts becomes of very great importance in as- 

 certaining the natural relations of animals. 



I think, therefore, to return to our subject, that the bones 

 from the Stonesfield slate, published under the names of Di- 

 delphis Prevostii and Did. Bucklandii, have belonged to 

 mammiferous animals, very nearly approaching the didelphs, 

 but of a distinct genus. 



Not having had the advantage of inspecting the portion of 

 a jaw preserved at "l'Ecole des Mines," I have been unable 

 to treat of that fossil in this memoir. 



M. Agassiz, who regarded these animals as of an ambigu- 

 ous nature among Vertebrata, has proposed for a generic 

 name that of Amphigonus. 



M. de Blainville, adopting the same views, without being 

 aware of the name proposed by M. Agassiz, which is not 

 cited in his note in Bronn and Leonhard's Journal, has pro- 

 posed that of Amphitherium or Heter other ium. As in all 

 that we can deduce from a study of the portions of jaws sub- 

 mitted to our examination, I see nothing which indicates an 

 ambiguous or heterogeneous nature, — and as the names pro- 

 posed by these naturalists express doubts which in my opin- 

 ion no longer have any foundation, I think it would be advis- 

 able now to apply a more significant appellation. I do not 

 think that sufficient time has elapsed for the ill consequences 

 to arise which generally follow changes of names in Natural 

 History, because those which I propose to replace by others 

 have not yet been adopted by systematic writers, and conse- 

 quently have not yet received the sanction of naturalists in 

 general. The name of Thylacotherium appears to me a pre- 

 ferable one. 



If we call to mind the figure of the fossil jaw published by 

 Mr. Broderip, which is taken from a fragment that I have not 

 examined, the new genus of fossil Mammalia will have 

 the following characters, taken from the examination of the 

 lower jaw only. 



Eight incisor teeth, two canines, and ten molars, with five 

 or six false anterior ones ; the hinder teeth presenting a sum- 

 mit consisting of five tubercles, three internal and two exter- 

 nal, the latter succeeded by a small "talon." 



The two species referable to this genus, are — 



Thylacotherium Prevostii, [Didelphis Prevostii, Cuvier), 



