ORDINAL TYPES OF MOLARS: MARSUPIALIA 109 



Primitive Tuberculo-sectorial Types. 



Bensley shows that the Oligocene opossum (Pemtherium) molar may 

 be taken as the theoretical starting point from which development took 

 place along two main lines, namely (1) the carnivorous and (2) the omni- 

 vorous and herbivorous. 



If we arrange the teeth of the Australian Marsupials according to the 

 analogy with the Placentals, we obtain such a result as is shown diagram- 

 matically in Figure 58, Nos. 1, 2. The primitive tritubercular, tuberculo- 

 sectorial type is here represented by the teeth of Daxyurus vircrrinnx 

 (58 r?), but would be still more strikingly illustrated by the teeth of one 

 of the purely insectivorous forms of the Dasyurida? (Sminthopsis, Ante- 

 chinomys, Phascologale}. All these Marsupials exhibit molar teeth which 

 may be readily compared in pattern cusp for cusp with those of 

 Didelphys. 



Carnivorous Marsupials. 



From this archetype, so to speak, the first line is the carnivorous line 

 to be compared among the Placentals with the specialized Creodonta 

 (Oxycena, etc.). Among Marsupials the carnivorous evolution is entirely 

 confined to the single family Dasyuridae, and it culminates in the teeth 

 of the Tasmanian wolf (Thylacynus cynocephalus). The teeth of this 

 animal are represented in Figure 58, g\ they show all the essential 

 characters of the teeth of Dasyurus except that in the lower teeth the 

 metaconid is absent (this cusp also disappears in the lower cutting- 

 teeth of the Felidse among Placentals). The progressive carnivorous 

 adaptation is effected in the upper molars by a conversion of the 

 paracone and metacone and metastyle into cutting blades and by a 

 degeneration of the other styles, which become reduced to very incon- 

 spicuous tubercles. In the lower molars the corresponding cutting blades 

 are formed by the elevation and lateral compression of the paraconid, 

 protoconid and hypoconid, while at the same time the cusps on the 

 other side of the crown, the metaconid, hypoconulid and entoconid are 

 either reduced or disappear (metaconid). 



Omnivorous and Herbivorous Marsupials, 



Turning to the second group, which leads to the omnivorous and 

 herbivorous adaptations of the teeth, we find that the first stage is 

 represented in the molars of the Bandicoots (Peramelidas) Fig. 58c, 59. 

 Analogy with the evolution of these teeth is to be sought among the 

 primitive insectivorous-omnivorous Placentals, c.<j. among the modern 

 Insectivora, in Tupaia and Myogal:. 



The main trend is toward cusp addition ; thus the upper molars 



