im DE. B. A. BENSLEY ON THE EVOLUTIOX 



Jiiember of the second line, which, in all probability after the cmnivorous stage, has 

 become carnivorous instead of herbivorous. In view of the general sequence in the 

 Australian group, this development is to be looked upon as an aberrant one. Analogous 

 cases of perverted habit and dental adaptation are to be found in the Placentalia, a con- 

 spicuous case being afforded by the fish-eating rodent Ichthyomys (Thomas, 1 893) of South 

 America, in which the herbivorous principle of the Rodentia has not been adhered to. 



The presence of enlarged posterior premolars in Thylacoleo points to a former condition 

 of at least incipient herbivority. The excessive enlargement and ungrooved character of 

 the teeth represent more special carnivorous adaptations, repeated in the placental 

 Carnivora, but without parallel in marsupial carnivores. 



The reduced character of the upper canines possesses a similar significance. In the 

 smaller members of the Dasyuridae the canines are moderately developed, and following 

 the carnivorous evolution they show a successive increase in functional importance. In the 

 Phalangeridai we find the smaller members presenting well-developed upper canines, but 

 followina' the omnivorous-herbivorous evolution there is a successive decrease in functional 

 importance, and in derived families total obliteration. Thylacoleo is quite as advanced in 

 the reduction of the canines as Phalanger or Trichosurus. 



In Thylacoleo the piercing elements of the dentition are the median upper and lower 

 incisors. It is interesting to note that in the Dasyuridaj a piercing modification of the 

 upper incisors appears as a prototypal insectivorous character, and that in the carnivorous 

 evolution the piercing function is taken over by the canines. In the more primitive 

 Phalangerida; we find both upper and loAver median incisors modified as piercing-organs ; 

 while in more advanced forms the lower teeth assume a cutting character, atid the upper 

 become slightly reduced. It is difficult to say whether the incisors of Thylacoleo were 

 formerly modified as cutting elements ; but it is apparent that the animal must have 

 developed along normal phalangerine lines to such an extent that only the median 

 incisors were left as possible piercing-organs. 



The reduction of the molars, although repeated in the most sjiccialized of the placental 

 Carnivora, contrasts strongly with the sectorial elaboration met with in the typically 

 carnivorous Dasyuridae, and has probably resulted from the presence iu the ancestral 

 form of inadaptive quadrituberculate bunodont patterns as much as Irom the sectorial 

 elaboration of the posterior premolars. 



The Adaptive Modifications of the Foot-structure in the Australian- 

 Marsupials. 



On comparing the general types of foot-structure in the Placentalia it is seen that 

 certain forms, moie especially some of the Creodonta, present an extremely close 

 approximation to the ideal pentadactyl type, which we assume to represent the starting- 

 point for the foot-evolution of the Mammalia generally, and, further, that the great 

 majority of the remaining forms present advanced modifications of this type bearing a 

 decidedly terrestrial stamp. The predominance of terrestrial modifications and the 



