352 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [June 4, 



of developed incisors are approximated and placed collaterally, as in 

 the placental Eodents ; and commonly they are projected forwards 

 with but a very slight upward inclination. They are unequally op- 

 posed in the upper jaw by two or more incisors on either side. Why 

 there should be this plurality of incisors above, and only two inva- 

 riably occupying the same position below, is wholly unknown to us ; 

 but the constancy of the structure makes it certain that there must 

 be a sufficient cause for it in nature ; and we employ the generaliza- 

 tion, empirically arrived at, with as much confidence as we do the 

 law of necessary correlation*. In many critical cases, where the 

 evidence is limited or defective, the empirical is even a safer 

 guide than the rational law, since it is freer from the risk of errors 

 of interpretation. Applied to the instance before us, it is manifest 

 that the principle on which the incisors in Plagiaulax are framed, in 

 regard of number, order of suppression, collateral position, and re- 

 lation to the premolars, corresponds exactly with the type of the 

 Marsupial Herbivores, such as Halmaturus, Hypsiprymnus, and PTias- 

 colarctus, and that it is wholly at variance with the Carnivorous type. 



Let us now test the opinion in its professed character as a physio- 

 logical deduction. Throughout the Mammalia, where teeth perform 

 the functions of canines, '' to pierce, retain, and kill," they are held 

 well apart through the interposition of a line of incisors, — the end 

 being obvious : the points of penetration are doubled, the grasp is 

 strengthened by vddening the base, and the dilacerating and killing 

 powers are multiplied. To arrange them collaterally in the axis 

 would be to place them at a disadvantage to the end to be attained. 

 But when a gnawing power is required, the middle incisors are power- 

 fully developed, and placed collaterally in the axis of the jaws, one 

 on each side, above and below, as typically exemplified in the pla- 

 cental Rodents and Cheiromys. Doubtless, a Eat when seized can 

 inflict a smart wound on the hand : but the power is a secondary at- 

 tribute, complementary to the main function. Regarded in this 

 aspect, it is negatively stamped upon the incisors of Plagiaulax by 

 their collateral position, that they are not constructed upon the Car- 

 nivorous plan of design, nor in rational correlation thereto. 



It is obvious that this position of the teeth in Plagiaulax was not 

 overlooked by the author of ' Palaeontology;' for, on the first oc- 

 casion, he describes the incisor of P. BecJclesii as being " very large, 

 shaped like a canine, but implanted by a thick root in the fore-part 

 of the jaw, like the large lower incisor of a Kangaroo t or "Wombat." 

 But the shape of the tooth prevailed in deciding him to pronounce it 

 carnivorous. Now, the form differs in the two species : and I ask 

 any Comparative Anatomist to look at fig. 15 of my former commu- 

 nication (p. 281), and say whether the tooth there represented is 

 formed to pierce, retain, and kill — being the attributes with which 

 Professor Owen invests the incisor of P. Becklesii. It is projected 

 forwards with a slight upward inclination, somewhat as in the vege- 



* Cuvier, ' Discours Preliminaire,' p. 51. 



t Encyclop. Brit., 8th edit. vol. xvii. p. 161. " Shrew and Wombat " are sub- 

 stituted in the ' Palaeontology,' p. 353. 



