354 PKOCEEDINGS OF THE GEOLOGICAL SOCIETY. [June 4, 



been broken away from the fore-part of the symphysis. The upper 

 and fore-part shows the alveolus and base of a tooth (pi. 11. fig. 3, c) 

 which has projected obliquely upward and forward. It is separated 

 by an interspace of 3 lines from the sectorial, and would seem to be 

 the sole tooth in advance of it. If the ramus be really produced at 

 the upper part of the symphysis further than is indicated by the 

 present cast, it may have contained one or more incisors, and the 

 broken tooth in question may he the lower canine. If, however, this 

 be really the foremost tooth of the jaw, it would appear to be one of 

 a pair of large incisors, according to the Marsupial type exhibited by 

 the Macropodidce and Phalangistidce " *. " But in the lower jaw the 

 carnassial is succeeded by two very small tubercular teeth, as in 

 Plagiaulaoc ; and there is a socket close to the symphysis of the lower 

 jaw of Thylacoleo, which indicates that the canine may have termi- 

 nated the dental series there, and afforded an additional feature of 

 resemblance to the Plagiaulax " t« 



In all this, it will be seen, the argument is within the domain of 

 conjecture ; the tooth oscillates between canine and incisor ; and not 

 merely so, but the principles which are followed as guides in this 

 walk of investigation are set aside, to give place to the illusory in- 

 dications of mutilated external form. If the tooth represented by a 

 stump or socket proves to be a canine, the comparison will not hold ; 

 but if it be solitary with the position of an incisor, will it even then 

 bear out Professor Owen's hypothesis, that Thylacoleo, which he in- 

 fers to have been one of " the fellest and most destructive of preda- 

 tory beasts $," may have had the laniary portion of its teeth in the 

 lower jaw constructed on the type of the most meek and defenceless 

 of herbivorous marsupials ? Bearing in mind 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 

 conjunction of characters assumed above be hereafter estabhshed, 

 there are theoretical considerations which would prove to demonstra- 

 tion that the types of construction are still absolutely distinct. Por 

 in the supposed case the outermost incisor would be the one deve- 

 loped, the inner ones being suppressed ; while, conversely, in the 

 Macropodidce. 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 palasontological investiga- 

 tions 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 principles, which provide the corrective. 

 I need hardly add that the argument drawn from Thylacoleo has, in 

 my view, no bearing on the incisors of Plagiaulaoc, 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 



^ PMl. Trans., toI. cxlix. p. 318. t Palseontology, p. 432. 



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



