I S S 2 .] Geology and Paleontology. 5 2 1 



as that which has taken place in the history of the descent of the 

 Carnivora. Ctenacodon, as having the largest number of premo- 

 lars, which have the least amount of sculpture, is the least special- 

 ized of all the genera. Thylacoleo, with the rudimental character of 

 the true molar teeth, is the most specialized, as it is the latest in 

 time. The Macropodida retain the full series of true molar teeth 

 of the primitive Mammalia, and present only a cutting third pre- 

 molar in the lower jaw, the fourth resembling the true molars. 

 Thus the cutting tooth of Thylacoleo is not the homologue of the 

 cutting tooth of Hypsipryjnuus as supposed by Professor Flower; 1 

 since the latter corresponds with the cutting tooth of Ptilodus, 

 which is the fourth premolar of Plagiaidax. We must therefore 

 regard Hypsiprymnus as the descendent of a type from which the 

 id<B were also derived, in which some of the premolars, 

 as far as the third only, were trenchant, and in which the fourth 

 premolar possessed the tubercular character of the true molars. 

 Such a type would belong to Jurassic and perhaps even to Trias- 

 sic times, and might well have continued to the Eocene. I call 

 it provisionally by the name Tritomodon. The lines of the de- 

 scent will appear as follows : 



Tritomodon (theoretical). 



Ctenacodon \ 



Plagiaulax 



Ptilodus ' 

 Catopsalis 



\ Hypsiprymnus 

 Thylacoleo \Macropus 



The discussion between Professor Owen, on the one hand, and 

 Messrs. Falconer, Krefft, and Flower, on the other, as to the 

 nature of the food of Thylacoleo, is known to palaeontologists. 

 *rom the form of the teeth alone Professor Owen inferred the 

 carnivorous nature of the food of this genus, while his opponents 

 interred a herbivorous diet from the resemblance between the 

 • »* ition and that of the herbivorous Hypsiprymnus. As the re- 

 sult of the discussion affects, in some degree, the genera Catopsa- 

 lls .^ Ptilodus, I recall it here. The comparison of Thylacoleo 

 ■■anus is weakened by two considerations : First, the 

 act that the cutting tooth of the former is not homologous with 

 e cutting tooth of the latter; and second, that the grinding 

 nes of molars of the former is rudimental, while in the latter it is 

 °mplete. It evidently does not follow that because Hypsiprym- 



Quarterly Journal Geological Society, 1S68, Vol. XXIV, p. 307. 



