358 PROCEEDINGS OP THE GEOLOGICAL SOCIETY. [Juiie 4, 



organ, without reference to affinity, as, for instance, among the 

 Mammalia, in the canine of Machairodus and of the Musk-deer. 

 Thylacoleo and Plagiaidax may be regarded as being as wide 

 apart among the Marsupials as the two former are among Placental 

 Mammals. The solitary trenchant premolar in some of the species 

 of Hypsiprymnus is said to attain a very large development. We 

 have the authority of Professor Owen for the statement, that in two 

 Potooroos of New Guinea its antero -posterior extent nearly equals 

 that of the three succeeding molars *. If the teeth of Thylacoleo and 

 Plagiaulax had been on the same morphological plan of construc- 

 tion, the agreement in the number of molars would clearly have 

 carried weight ; but, as such does not appear to be the case, the co- 

 incidence ought not to overrule the other indications, more especially 

 as the form of the crowns of the molars in the two genera is totally 

 different. In Thylacoleo, the first tubercular tooth has the crown 

 compressed, supporting two cusps on its axis, the anterior lobe being 

 more or less conical, with a smaller lobe behind it, both on the usual 

 carnivorous type of construction. The second tubercular is only 

 known through its socket. In both species of Plagiaulax, the two 

 molars present oblong crowns, supporting two opposed lines of mar- 

 ginal eminences, separated by a depression. In my original descrip- 

 tion, I referred to the fact that in Dromicia and Acrohata the molars 

 are reduced from the ordinary number, four, to three. In Plagiaulax 

 the suppression is carried still further, two only being developed. 

 The agreement in this respect between the latter and Thylacoleo does 

 not impress me with the idea of affinity, although admitting, as I do, 

 that it ought to be duly weighed. 



I have entered in such detail upon the dental characters, because, 

 by the consent of all observers, they are of paramount weight in the 

 solution of a question of this nature. If the type be distinctly indi- 

 cated by them to be herbivorous or carnivorous, the other charac- 

 ters, however modified they may be, will ultimately be found to be 

 in relation to the teeth. The author of ' Palaeontology,' having 

 formed his opinion on the teeth, then examines the characters of the 

 lower jaw and finds them in conformity. He adduces the shortness 

 of the horizontal ramus in proportion to its depth as indicative of 

 robustness; also the broad and high eoronoid process, and the peduncu- 

 late condyle placed below the level of the grinding teeth (above, 

 p. 349). They are all regarded as proving a carnivorous type. They 

 were not overlooked in my former communication : — " The charac- 

 ters of the jaw are so peculiar, and in some respects of so mixed and 

 complex a nature, that they ought to be weighed with caution, in 

 conjunction with the teeth, in forming any opinion of the affinities of 

 Plagiaulax. The low position of the condyle is so pronounced, and 

 the elevation of the eoronoid above it so considerable, that, regarded 

 per se, supposing no teeth had been discovered, they might have been 

 considered to imply with some degree of certainty a predaceous ani- 

 mal"t. But there were other characters, which, taken in conjunction 



* Odontography, vol. i. p. 389. 



t Quart. Journ. Geol. Soc, vol. xiii. p. 273. 



