1865.] Prof. Owen on the Skull of Thylacoleo carnifex. 343 



another effect of air, namely, fluid friction, the coefficient for which they 

 believe to be independent of the tension ; and as far, therefore, as this effect 

 is concerned, little is gained by diminishing the amount of the residual 

 air. It would appear, however, that the fluid friction of hydrogen is much 

 less than that of atmospheric air ; so that, were the heating effect due to 

 fluid friction, it ought to be less in a hydrogen vacuum. An experiment 

 was made with this purpose ; and, other circumstances being precisely simi- 

 lar, it was found that in a hydrogen vacuum the heating effect due to rota- 

 tion was 22*5, while in an air vacuum it was 23'5. These numbers may 

 probably be considered as sensibly the same, and this experiment would 

 therefore appear to denote that the effect is not due to fluid friction. 



15. The authors, in submitting these remarks to the Eoyal Society, do not 

 suppose that their experiments have yet conclusively decided the origin of 

 this heating effect, but they hope by this means to elicit the opinions of 

 those interested in the subject, which may serve to direct their future 

 research. 



VIII. " On the Fossil Mammals of Australia. Part II. Description of 

 an almost entire Skull of Thylacoleo carnifex, Ow." By Professor 

 OWEN, F.R.S., &c. 



(Abstract.) 



In this Part the author gives additional cranial and dental characters of 

 the extinct marsupial carnivore, Thylacoleo, deduced from examination 

 of better-preserved fossils, obtained from freshwater deposits in Darling 

 Downs, Queensland, Australia. 



The forepart of the skull, wanting in the first-described specimen from 

 similar deposits in the province of Victoria, is preserved in the present 

 specimen, showing the premaxillary bones, which are relatively larger than 

 in placental felines. Each bone has three teeth, of which the foremost 

 is developed into a tusk, the second and third being very small. There 

 is no canine, or no tooth developed as a laniary in the maxillary bone. 

 In the short extent of the alveolar border of this bone between the great 

 carnassial molar and the maxillo-premaxillary suture, there are two approxi- 

 mate small round sockets, which lodged either one double-rooted tooth or 

 two small single-rooted teeth. But dental development has mainly ex- 

 pended itself upon the perfection of a pair of laniary incisor tusks, in 

 both upper and lower jaws, for piercing, tearing, and holding, and a pair 

 of carnassials in both jaws for flesh-cutting. These, in the present speci- 

 men, closely agreed with those described in the former one, but were more 

 worn : they are the largest examples of these peculiarly modified shear- 

 blade teeth in the mammalian class. Although the tusks are incisors not, 

 as in placental carnivora, canines they possess, through the singular short- 

 ness of the facial part of the skull in Thylacoleo, the same mechanical 

 advantage, in their proximity to the biting-power of the enormously deve- 



