163 



of mutual continuous attrition. In Rodents, whether placental or marsupial, the oblique 

 surface of wear or use in both upper and lower pairs of the large front teeth has sug- 

 gested the comparison with the chisel, and the term " scalpriform." Such incisors have 

 a trenchant margin as the human incisor has ; but the superadded sloping surface of 

 attrition in the Rodent indicates the continuous as distinguished from the occasional 

 application of such front teeth. The Lemurine Aye-aye presents the same character as 

 a guide to the inference of function of incisors, and at once exemplifies the difference of 

 such function and that of the homologous pairs of pointed unworn teeth in Tkylacoleo 

 and Plagiaulax. 



It is proper, in pursuing comparisons for the purpose of arriving at truth, that, 

 besides the front view of the incisors of the Koala*, we should contrast their working 

 surface (fig. 6, i) with that in the corresponding teeth of Thylacoleo. A comparison of 

 Cut, figure 5, i with figure 3, Plate IX., will show that the one has the continuous or 

 frequent action, the other the intermittent and occasional. It is evident that the six 

 incisors of the upper jaw, as well as the lower pair, in the Koala, work much and 

 continuously in cropping and gnawing off the vegetable food which the large, nume- 

 rous and complex grinders (fig. 6, m) pound to pulp for the bolus of deglutition. 



A minor but sufficiently conspicuous degree of attrition characterizes the narrower 

 upper and the lower procumbent incisors of the Bettongs and Rat-Kangaroos. 



In the Bettongia penicillata, with such worn incisors and with all the molars in place 

 and showing habitual use, the trenchant premolar retains its vertical groovings to the 

 cutting-edge of both the outer and inner sides. They have been used to divide the 

 grass-blade or the leaf-stalk, or other tough part or fibre of the vegetable food ; but the 

 more important and continuous work of mastication has had grinders in number, size, 

 massiveness, and complexity of horizontal area fitted to perform it. Old age is attended 

 with seeming exceptions to this rule in both human incisors and hypsiprymnal premolars, 

 which then show the wear or work of a life. 



Independently of the correlative guide, the worn surfaces of the Thylacoleo s carnassials 

 show, like those of the Lion's, and like the scalpriform incisors of the Rodents, that their 

 work and office were of the continuous kind ; which, with their shape and position in 

 the jaw, was for flesh-cutting, not for wood-cutting, or leaf-cutting or grass-cutting ; for 

 the succeeding few and small tuberculars could do nothing to the purpose with slices of 

 such vegetable substances. 



How far this deduction of function from mere form may be "facile" or "arbitrary" 

 it is not for me to say ; but it by no means authorizes any one to infer, because the 

 correlation of the premolars of Thylacoleo and Plagiaulax with few and small tuberculars 

 and large laniaries favours their carnassiality, that " the premolars of Hypsiprymnus 

 ought also to be carnivorous "f. All that the mere form of that tooth shows is, 

 that it cuts. What manner of substances were so cut can be inferred from the asso- 



* Exclusively given by Professor Flower in his advocacy of the herbivority of Thylacoleo, XII. p. 313, fig. 4. 

 t X. p. 357; XI. p. 440; also XII. p. 318. 



