161 



latter to predatory actions, and is consequently and concomitantly associated with a dif- 

 ference of form of the entire mandible : that part in Plagiaulax conforms with the 

 lower jaw in Sarcophilus and Thylacoleo in as marked a degree as it differs from the man- 

 dible in Chiromys, in placental Rodents, in Phascolarctos, and in Hypsyprimnus. 



Comparison of Incisors of Di_protodont Paucidentata with those of Chiromys and 

 Bodentia. — Tliylacoleo and Plagiaulax, it is affirmed, " agree with Chiromys in the colla- 

 teral position and upward direction of their strong incisors "*. Doubtless; but they differ 

 in the character of the terminal surface indicative of the kind of work to which those 

 incisors were respectively put in Thylacoleo and Chiromys. Admitting the Aye-aye to be 

 " as rare and aberrant among existing MammahVf as are Thylacoleo and Plagiaulax 

 among fossil Marsupials, yet the Aye-aye shows on its lower front teeth a long smooth 

 sloping surface, the result of the scraping, cutting, chisel-like action of the opposed scal- 

 priform incisors J. 



Thylacoleo and Plagiaulax preserve the pointed termination of the lower incisors, or 

 if they be blunted or broken, they show no signs of habitual attrition. Chiromys com- 

 bines a compressed form with peculiar fore-and-aft breadth of the incisor, which has its 

 thick enamel limited to the front border and to a contiguous portion of the sides, but 

 coextensive in length with the deeply implanted tooth. The lower incisors of Thylacoleo 

 and Plagiaulax have the proportion of transverse to fore-and-aft breadth, and the con- 

 tinuous sheath of enamel (Plate XIII. fig. 8) limited to the exposed crown, which are 

 characteristic of the laniaries in Potamogale and Felis : consequently the crown or ex- 

 posed part of the long and large incisor of Thylacoleo and Plagiaulax is that alone which 

 is curved, and the division into crown and root is recognizable. The entire scalpriform 

 incisor of Chiromys, like that of true Rodents, is curved in the segment of a circle § 

 and the tooth (fig. 9, i) preserves its diameters of depth and breadth from the end of 

 the worn, sloping, eroding surface of the crown (i) to the base of the implanted part, 

 and this part is much longer, with a concomitantly longer socket, than in Thylacoleo 

 and Plagiaulax. The above-defined broad and striking differences between the lower 

 incisors of Chiromys and those of Plagiaulax and Thylacoleo militate strongly against 

 the conclusion of Plagiaulax and Thylacoleo being Marsupial forms of Rodent, or 

 "Rodent types of Marsupial" ||, and are decisive against the alleged "clear evidence of 

 their phytophagous and rodent plan of construction"^". 



Dr. Falconer pursues his argumentation as follows : — " Let us now consider Owen's 

 inference as to the function of these teeth. It is expressed thus: ' The large front tooth 

 is formed to pierce, retain, and kill ; the succeeding teeth are like the blades of shears, 

 adapted to cut and divide soft substances like flesh,' &c. Professor Owen has else- 

 where described the premolar of Hypsiprymnus as trenchant**, and I have shown above 



* X. p. 364 ; XI. p. 449. t Id. ib. + Owen, < On the Aye-aye,' &c, pi. 20. fig. 3. 



§ " The incisors are long, large, much compressed, regularly curved in segments of equal circles, the upper 

 pair describing one-fourth, the lower pair one-half of such circle." — Owen, op. cit. p. 55. 



|| X. p. 349 ; XI. p. 431. f X. p. 353 ; XI. p. 436. ** Odontography, vol. i. p. 389. 



