1862.] FALCONER PLAGIAULAX. 351 



to rest his physiological deductions^ were sound, the demonstration 

 would be complete. They are put together with an exemplary show 

 of harmony, and, with a single exception, every link in the chain is 

 supphed. But there are, in the case, considerations of paramount 

 import in an argument of this nature, that lead me to question their 

 soundness, and to dissent from the conclusions. 



And first, as regards the admitted facts. Professor Owen agrees 

 that the Purbeck remains establish two species of Plagiaulaoo ; and, 

 as he has adopted two of the wood-cuts given in my original descrip- 

 tion of these species, it is presumed that the correctness of the figures 

 is not questioned. The marsupial nature of the forms is not dis- 

 puted, nor is there any difference of opinion about the number or 

 designation of the teeth. 



In both species there is a solitary incisor on each side of the lower 

 jaw, in the fore part of the incisive border, closely followed, without 

 the interposition of a canine, by a series either of three or of four 

 premolars. The rami converge to a narrow point in front, so that 

 the tooth occupies the entire width of the incisive border on each side ; 

 and fig. 13, p. 280, of my former communication, representing the 

 symphysial portion endwise, shows (what is confirmed by the other 

 figures) that the two incisors were approximated and collateral, as 

 in the rodent type, placental or marsupial. In P. minor, fig. 15, the 

 tooth is procumbent. In the other and larger species, P. BeckUsii, it 

 is more robust, with a thicker root, and with a more decided curvature 

 upwards, suggesting, at the first sight, some resemblance to the form 

 of a canine. In both species the point is bevelled* ; and I failed to 

 observe in either any mark of the play of an opposed upper tooth. 



What was the function of these incisors? Professor Owen's 

 opinion is expressed thus : '^ The large front tooth of Plagiaulax is 

 formed to pierce, retain, and kill." This conclusion arrived at, the 

 other characters are naturally regarded in unison with it, until the 

 genus is finally presented to us as a predaceous carnivore. It is 

 therefore necessary to examine the evidence closely, l^ow, in 

 solving a question of this kind, comparative anatomy supplies for our 

 guidance fundamental principles, which govern the interpretation of 

 mere form. Let us revert to the known marsupial genera, and see 

 what light generalized observation upon them throws upon the ques- 

 tion. In all the Carnivorous genera and species, fossil or recent, of 

 which the dentition has been accurately determined, there are three 

 or more incisors, followed by a canine, on each side of the jaw, above 

 and below ; and the empirically observed result is consistent with a 

 rational interpretation of the arrangement, in reference to their food 

 and the means of procuring it. On the other hand, in all the ex- 

 isting strictly phytophagous genera, there is only a solitary incisor 

 (being that next the axis) on either side of the lower jaw, and no 

 canine ; or if, as among the Phalangers, additional teeth are deve- 

 loped, the outer incisors and canine are alike rudimentary. The pair 



* Not in the sense of being denuded of enamel by wear ; but the posterior sur- 

 face is flattened near the apex, so as to yield a slightly bevelled point {op, cit. 

 p. 268). 



