Family DIDELPHIDAE 47 



small braincase, the inflected dentary bone, fig. 40b, and the 

 large number of teeth (total 50). The sutures between the skull 

 bones tend to remain open, and there is little fusion of bones 

 even in the oldest Individuals. Dental formula:* I 5/4, C 1/1, 

 Pm 3/3, M 4/4. 



Life History. — The opossum, an inhabitant of woodland, 

 fig. 1, attains sexual maturity at the age of approximately 1 

 year. The female may have one or, rarely, two litters annually, 

 the first usually in late February and the second in late July. 

 About 13 days after mating, she gives birth to 7 to 20 young, 

 each no larger than a honey bee. The young are nude, sight- 

 less, and scarcely more developed than small embryos. By their 

 own strength and instinct they make their \\ay from the birth 

 canal to the mother's pouch. The mother assists them only by 

 making a moist path through the abdominal fur with her tongue 

 and in sitting nearly upright so that the tiny babies will not lose 

 their way and become entangled in dry hair. When they get to 

 the pouch, each one seizes a milk-providing nipple on which it 

 will retain its hold for several weeks. There are only 13 nipples 

 in the pouch and, if the number of young is more than 13, the 

 superfluous number starve. 



When the baby opossums are 50 days old, they are about as 

 large as full-grown house mice. By the end of 2 months, their 

 eyes open, and then for the first time they release their hold on 

 the nipples. After 10 more days, they leave their mother's pouch 

 to clamber about her body, fig. 42, but remain nearby. When 

 about 80 days old, they are as large as Norway rats. At this 

 time, they go adventuring on the ground but still return to 

 their mother's pouch for nourishment. If the litter is large, they 

 must nurse in relays. When the young are about 3 months old, 

 the brood gradually breaks up, and each member wanders about 

 until it finds a suitable home in a hollow tree or log, or in a 

 woodchuck or skunk burrow. 



When winter comes, the opossum lines its nest thickly with 

 dry leaves or grass and spends the coldest periods dozing in the 

 warmth of a coat of fat gained from its autumn feasts. Only 

 when the nights are not so cold does it come forth to forage, 

 and then it soon hustles back to its den so that its bare ears and 

 tail do not become frostbitten. 



'An explanation of dental formula is given in the glossary. 



