11)10.] The Polypwtodontia. ' 201 



(2) The foot-structure and many other characters of this prototype are 

 reahzed in the existing Didelphichne, especially the smaller species of Mar- 

 mosa. The prototypal dentition is represented in tlie Oligocene Didel- 

 phid Peratheriuvi fugax. 



(3) Starting from this type the divergent lines of adaptation were 

 traced: as regards diet from insectivorous to carnivorous, omnivorous and 

 herbivorous types, and as regards modes of locomotion from arboreal to 

 terrestrial, fossorial, saltatorial and cursorial types. Bensley's conclusions 

 rest partly on the assumption that the tritubercular molar was prototypal 

 in the Marsupialia as well as in the Placentalia. His studies of the evolution 

 of the feet tended to confirm this assumption. 



A tabular summary of the morphogenetic and functional relations of 

 the Polyprotodont families (Fig. 14), based largely upon Bensley's observa- 

 tions is given below: 



Adaptive Radiation of ihe Polyprotodontia. 

 DIDELPHOIDEA Opossums, Dasyures, Thylacynes, etc. 



DiDELPHIID.E. 



Marmosa group. 

 Arboreal} 



Size small, primitively mouse-like. 



Tail primitively prehensile, scaly (non-prehensile, hairy in Per- 



cnnys). Pes 5-toed, digit I opposable, digits radiating. 



Pes eleutherodactylous - {Peramys, secondarily terrestrial) to 



incipiently syndactylous ^ (occasionally in Marmosa). 

 Plantar pads separate, transversely striate. 

 Gait plantigrade, primitively scansorial. 

 Insectn^orous. 



Polyprotodont (If). 



Molars simple, tritubercular; metacone enlarged, cusps pointed. 

 ? Peratherium, Marmosa, Peramys, Microbiotherium, Dromiciops, 

 Caluromys {Philander). 

 Didelphis group. 



Arboreal to terrestrial, 

 Size larger. 



Pes eleutherodactylous (digits separate, II — III). Other limb- 

 characters much as in preceding group. 

 Insectii'orous-otnnivorous-carnivorous. 

 Polyprotodont. 



Molars tritubercular but stouter, somewhat less piercing, with 

 long posterior blade. 



1 The words in italics refer to special habitat and diet. Under these headings are listed the 

 principal characters relating to locomotion and dentition. 



2 /. e., with all the digits remaining separate. 



3 /. e., with digits II and III conjoined. 



