354 VERTEBRATE ZOOLOGY 



that at one time the marsupials were more fully diphyodont and had 

 a true placental gestation. 



Regarding the marsupials as a single order, we may divide them 

 into two sub-orders: Polyprotodontia (many incisors), and Diproto- 

 dontia (two incisors) . The first group is now believed to be the more 

 primitive and the second more highly specialized and somewhat 

 senescent. 



SUB-ORDER I. POLYPROTODONTIA 



This group, which consists mainly of insectivorous and carnivorous 

 types, is more primitive than are the herbivorous diprotodonts. The 

 polyprotodonts are characterized by the possession of four or five 

 incisors on each side of the upper jaw and one or two fewer in the 

 lower jaw; both canines and molars have the typical carnivorous 

 shape. They are confined to Australasia, with the exception of the 

 American opossums. 



Family 1. Didelphidce (the Opossums). Of all living marsupials 

 the opossums appear to be the most generalized in both structure 

 and habits. The Virginia opossum (Fig. 184, A), Didelphys virginiana, 

 is the only North American member of the family and deserves spe- 

 cial mention. It is distinctly arboreal, with a prehensile tail adapted 

 for clinging to branches and for use as a hold-fast by the young, who 

 wind their tails about the arched tail of the mother. The opossum 

 is omnivorous, eating fruit, insects, birds, reptiles, and their eggs. 

 There is a distinct pouch in which the young are suckled and carried. 

 The animal is nocturnal in habit, sleeping in hollow trees during the 

 day. The death-feigning instinct has received the proverbial de- 

 scription " playing 'possum." Important genera of the family are: 

 Didelphys, Marmosa, Chironectes, Peramys, and Philander; there are 

 about twenty-five species, all American. Marmosa murina is a tiny 

 opossum about the size of a small rat; Chironectes is an aquatic type 

 with webbed feet and about the size of a muskrat. It is the only 

 aquatic marsupial (Fig. 184, D). 



Family 2. Myrmecobiidce (Banded Anteaters). This small family 

 is represented by a single species, Myrmecobius fasciatus (Fig. 184, B), 

 an animal about the size of a cat, with only slight specializations for 

 the anteating habit. Its snout is moderately prolonged ; its tongue is 

 very long and extensible and is covered with the customary sticky 

 secretion; the tail is covered with long, coarse hair; the claws are only 



