433 PLAGIAULAX. 



infers to have been one of ' the fellest and most destructive 

 of predatory beasts,' * may have had the laniary portion of its 

 teeth in its lower jaw constructed on the type of the most 

 meek and defenceless of herbivorous marsupials ? Bearing 1 

 in urind the sense in which the term ' type ' is accepted 

 among naturalists, I must avow that I have some difficulty 

 in realizing the conception. But should the unusual con- 

 junction of characters assumed above be hereafter established, 

 there are theoretical considerations which would prove to 

 demonstration that the types of construction are still abso- 

 lutely distinct. For in the supposed case the outermost in- 

 cisor would be the one developed, the inner ones being 

 suppressed ; while, conversely, in the Macropodidw it is the 

 innermost incisor which is developed, the outer ones being 

 suppressed. Morphologically, therefore, the types of con- 

 struction would be radically different. If palseontological 

 investigations were conducted in this manner, there would 

 be no limit to conjecture ; the landmarks which we profess 

 to follow would be disregarded, and disorder would face us 

 everywhere. But, happily, science furnishes unerring princi- 

 ples which provide the corrective. I need hardly add that 

 the argument drawn from Thylacoleo has, in my view, no 

 bearing on the incisors of Plagiaulax, and gives no support to 

 the carnivorous inference. 



Next, as regards the premolars. From their peculiar 

 characters and remarkable development, they furnish the 

 most striking features in the dentition of the fossil genus. 

 In P. BecMesii there are three, and in P. minor, four of these 

 teeth, which diminish rapidly in size from the last to the 

 first. 2 I here take the last as the most determinate in form, 

 and in its nature the most constant. I compared it rigor- 

 ously with the corresponding tooth of Hypsiprymnus Gai- 

 mardi, and I affirm now, as I did in my original paper, that 

 these homologous teeth, in the two genera, are identical in 

 every essential point of form and construction. In proof, I 

 refer to figures 5 and 6 of the representations in PL XXXIIL, 

 the former showing the last premolar of Plagiaulax, the latter 

 of Hypsiprymnus. The resemblance is so manifest and direct, 

 that I never contemplated that it could be called in ques- 

 tion ; but, as it has been questioned, it is necessary to de- 

 scend to particulars. In both, the crown viewed from the 

 side is of a quadrately oblong form, the length exceeding the 

 height ; in both, it is compressed and trenchant, the sides 

 sloping uniformly from the base to a thin edge like a wedge ; 

 in both, the basal part of the tooth presents a smooth sur- 



1 Phil. Trans., vol. cxlix. p. 319. 



2 See PL xxxiii. and sxxiv. — [Ed.] 



