Geological Society. * G5 



from the structure of the molares in any of the Phocse, 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 Phocidae, 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 Cuvier urged as 

 evidence of the generic distinction of the Stonesfield mammiferous 

 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 Blainville 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 

 Ferae, 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 formulae of tlie 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 

 teeth. From the occurrence of the above exceptions in recent pla- 

 cental Mammalia, the example of a like excess in the number of 

 molar teeth in the marsupial fossil ought rather to have led to the 

 expectation of the discovery of a similar case among existing mar- 

 supials, and such an addition to our zoological catalogues has, in 

 fact, been recently made. In the Australian quadruped described 

 by Mr. Waterhouse under the name of Myrmecobius an approxima- 

 tion towards the dentition of the Thylacotherium is exemplified, not 

 only in the number of the molar teeth, which is nine on each side of 

 the lower jaw in the Myrmecobius, but also in their relative size, 

 structure, and disposition. Lastly, with respect to the dentition, 

 Mr. Owen says it must be obvious to all who inspect the fossil and 

 compare it with the jaw of a small Didelphys, that contrary to the 

 assertion of M. de Blainville, the teeth and their fangs are arranged 

 with as much regularity in the one as in the other, and that no ar- 

 gument of the Saurian nature of the fossil can be founded on this 

 part of its structure. 



With respect to M. de Blainville's assertion that the jaw is com- 

 pound, Mr. Owen stated, that the indication of this structure near 

 the lower margin of the jaw of the Thylacotherium is not a true 

 suture, but a vascular groove similar to that which characterizes 



Ann. Nat. Hist. Vol.3. No. 14. i^fcrcM 839. f 



