294 



ANATOMY OF VERTEBRATES. 



234 



Dentition of lower jaw, Plagiaulax. 



carnassials ; development was concentrated on these at the cost of 

 the rest of the normal or typical dental s'eries. The foremost 

 teeth seized, pierced, lacerated or killed, the carnassials divided 

 the nutritive fibres of the prey. 



Thylacoleo exemplifies the simplest and most effective dental 

 machinery for predatory life known in the Mammalian class. It 

 is the extreme modification, to this end, of the diprotodont type 

 of Marsupialia. The skull exhibits all the concomitant carnivo- 

 rous modifications, in a like extreme degree. 1 



It is interesting to note that, just as the exceptional modifica- 

 tion of the polyprotodont type, in the modern Myrmecobius, was 



manifested by Amphitlierium in 

 Oolitic times, so likewise was the 

 zoophagous diprotodont modifica- 

 tion ; but on a smaller scale than 

 in Thylacoleo. The lower incisor 

 in Plagiaulax, fig. 234, i, was a 

 large, upcurved, pointed tusk : the 

 carnassial, p. 4, was of great fore- 

 and-aft length, coupled with nar- 

 rowness, and an oblique cutting edge, rendered sub-serrate by the 

 better-marked and more oblique lateral grooves, than in Tliyla- 

 coleo. Anterior to the carnassial, p, 4, there are two or three 

 similar and smaller sectorial premolars, in Plagiaulax, more of 

 the general diphyodont type being retained in the older zoopha- 

 gous diprotodont. Behind the carnassial are two small tubercu- 

 late molars, m 1, m 2, as in Thylacoleo. Some Palaeontologists, neg- 

 lecting Cuvier's guide-post of the true molar as the light-giving 

 tooth, have been led astray in regard to the affinities of Plagi- 

 aulax, referring it to the ( poephagous Potoroos and Kangaroos,' 

 which combine with a single trenchant and grooved premolar, four 

 large and massive grinders, of quadricuspid or transversely ridged 

 structure. 



C. Rodentia. In different orders of the placental as in the 

 marsupial diphyodonts there are instances in which the ordinary 

 number of incisors is diminished, and their growing power trans- 

 ferred to a single pair of tusks projecting from the forepart of the 

 upper or the lower jaw, or of both. The Dinotheres, Toxodons, 

 Mastodons, and Elephants, among the Ungulata, the Dugong in 

 the Sirenia, the Aye-aye in the Quadrumana, are instances of 

 this modification, which reaches its extreme in the latter mammal 

 and the elephants. In numerous Lissencephala a single pair 



1 cxix". 



