THE OPOSSUMS. 



195 



mals enter seriously into competition with the 

 members of this group. In captivity marsu- 

 pials generally appear dull-minded creatures, 

 without curiosity and without interest regard- 

 ing their surroundings. Neither towards man 

 nor towards the companions of their cage do 

 they show the slightest attachment. Most of 

 them are characterized by an astonishing ten- 

 acity of life. Among the Americans the opos- 

 sums have become proverbial on that account. 



From the Old World, which was formerly 

 peopled by marsupials, this group has dis- 

 appeared since Miocene times; at the present 

 day this old and singular group survives only 

 in Australia and America. In the latter area 

 the group is represented only by two peculiar 

 genera. All the other marsupials belong to 

 the Australian region, most of them to the 

 vast island itself, the others to the neighbour- 

 ing islands of New Guinea, the Moluccas, and 

 the Philippines. With the exception of the 

 dingo, a few rodents, a few bats, and the 

 monotremes, the whole Australian fauna con- 

 sisted at the time of its discovery of marsu- 

 pials, which, as we shall afterwards see, form 

 the oldest stock of the entire class of the 

 Mammalia. 



We arrange the marsupials according to 

 their mode of life as indicated by the structure 

 of their teeth and feet, and begin with the 

 American genera. 



THE OPOSSUMS 



(DIDELPHYIDA). 



The numerous species of this exclusively 

 American family are found from Patagonia to 

 Canada. In the New World they play in a 

 measure the part of the insect-eaters, which 

 are poorly represented in North America and 

 in South America not at all. They form 

 only two genera, the True Terrestrial Opos- 

 sums (Didelphys), and the Aquatic Opossums 

 or Yapocks (Cheironectes), which are confined 

 to South America, and resemble the otters in 

 their mode of life. 



The opossums are small nocturnal animals, 

 which attain at most the size of a cat, but are 

 frequently no bigger than a rat or a mouse, 

 which latter they resemble moreover in their 

 general habit. The longish head is pointed, 

 the muzzle set with strong whisker-hairs, the 

 eyes rather small, the ears, on the other hand, 

 pretty large and almost naked. The body is 

 pretty long and rests on short legs. It ends 

 in a long almost naked tail, capable of being 

 rolled up, and mostly scaly like that of the 

 shrews. Almost all the species make use of 

 this tail, like the American monkeys, to attach 

 themselves to branches of trees whenever they 

 wish to do so. The feet are five-toed, the 

 toes themselves free and provided with strong 

 hooked claws. The animals lay the whole 

 sole of the foot on the ground in walking. 

 The great toe of the hind-foot is very strong, 

 very long, and, as in the monkeys, completely 

 opposable. On account of the structure of 

 this hind-hand, so to speak, these animals 

 have sometimes been designated by the name 

 of the " hand-footed " (Pedimana). 



The dentition answers to the general char- 

 acter of that of the marsupials, inasmuch as it 

 comprises a large number of teeth, and more- 

 over exhibits a remarkable commingling of 

 teeth of a carnivorous with those of an insec- 

 tivorous type. In each half of the premaxilla 

 there are five recurved pointed incisors, the 

 middle one of which is usually the largest. 

 In the lower jaw we find only four such teeth. 

 The large recurved sharp-pointed canines re- 

 semble those of the fox. In each half of each 

 jaw there are, in addition, both above and 

 below, three compressed cutting premolars, 

 each with a double root and triangular crown. 

 So far this kind of dentition might pass for 

 that of a typical carnivore, but in each half of 

 each jaw there are, over and above all these 

 teeth, four true molars, which all exhibit the 

 characters of insectivorous teeth, and especially 

 so in the upper jaw, where the grinding surface 

 presents the characteristic appearance of a V, 

 the angle of which is somewhat obliquely 



