310 PKOCEEDINGS OP THE GEOLOGICAL SOCIETY. [April 8, 



" carnassial" of the true Carnivora is merely superficial is easily shown. 

 Indeed the differences between the upper tooth, with its simple 

 crown and " even and uniform " trenchant edge " describing a very 

 feeble concavity lengthwise," and the trilobed crown of the Feline 

 carnassial, and especially the absence of any distinct inner lobe or 

 tubercle supported by a third fang, have been so clearly pointed out 

 in the original description (Phil. Trans. 1859, p. 311) that little 

 more need be said on this point. There is another striking difference 

 in the lower tooth from that of the true Carnivora. As is weU seen 

 in the jaw in the Museum of the Eoyal CoUege of Surgeons, in which 

 the tooth is broken off rather below the base of the crown, it has 

 two roots, of which the anterior is considerably smaller than the 

 posterior, whereas in the lower sectorial tooth of the cats and hysenas, 

 these proportions are always exactly reversed. 



The differences which exist between these teeth of Thylacoleo and 

 their supposed representatives in the implacental carnivora are still 

 greater. Indeed there is no tooth, either of the upper or lower jaw, 

 of any of the Thylacines, Dasyures, or Opossums that can be with 

 any reason compared with them. ^Tien, however, we pass to ano- 

 ther group of the same subclass, the Hypsiprymni or Rat-kangaroos 

 (fig. 2, p. 312), we see at once in the great cutting premolar a miniature 

 of that of Thylacoleo. The position in the jaws, the shutting of the 

 upper tooth against the outer edge of the lower one, as shown by the 

 wearing and polishing of the respective surfaces, the general charac- 

 ter of the simple, compressed, trenchant croAvn, without any accessory 

 cusps or tubercles, and the relative proportions of the anterior and 

 posterior fangs of the lower teeth precisely correspond. 



On comparing a large sei'ies of skulls of Hypsiprymni of dif- 

 ferent species it will be found that this tooth, though always retain- 

 ing the above general characters, varies greatly in the details of its 

 configuration and in relative size. In some species it is much larger 

 in proportion to the size of the skull and the other teeth than in 

 others. In some the internal and external surfaces are nearly flat, 

 in others concave. The free margin in some is nearly straight, 

 in others (and this is more often the case) more or less concave. 

 Lastly, the vertical grooves and ridges on the sides of the crown, 

 found in nearly aU, vary greatly in number and character, being 

 numerous, fine, regularly parallel, deeply cut, and closely placed 

 in some, as H. Gaimardii and H. Grayii, or few in number and almost 

 obsolete, as in H. campestris, murinus, and rufescens. Among all 

 these modifications the corresponding tooth of Thylacoleo would easily 

 find a place, although it agrees exactly with no one existing species. 

 The vertical grooves and ridges, though distinct enough to show its 

 adherence to the type, are as little marked as in some of the exist- 

 ing species mentioned above. In relative size it considerably ex- 

 ceeds even that of H. rufescens or Dorcopsis Mulleri. 



To pass to the other teeth of Thylacoleo, the incisors in the upper 

 jaw are three on each side, known only by their sockets and 

 the broken stump of the first, which was clearly much larger 

 than the other two ; below there was a single large compressed inci- 



