CHARACTERS OF METATHERIA. 569 



sometimes fuse and form a csecum, which, according to 

 the degree of fusion, may be a single tube or divided by a 

 partition. Moreover, in Bennett's kangaroo, the CKCum 

 opens independently into the cloaca between the apertures 

 of the distal portions of the vaginae. 



The allantois is small, and does not unite with the sub- 

 zonal membrane. The yolk-sac is large and adheres to a 

 portion of the subzonal membrane. From this region in 

 some cases, e.g., opossums, villi are given off, which enter 

 into close connection with the glands of the uterine wall. 

 The unborn young seems to derive its nourishment from 

 the yolk-sac. 



The gestation is short, lasting only a fortnight in the 

 opossum, about five weeks in the kangaroo. Except in 

 some opossums, there is a marsupial pouch, usually with a 

 forward-directed aperture. Within this pouch are the teats, 

 and here too the delicate young are nurtured after birth. 

 The milk is forced down the throat of the young, the mam- 

 mary gland being compressed by the cremaster muscle 

 which covers it. 



Families of Marsupials. 



A. PoLYPROTODONTlA. " Incisors numerous, small, subequal. 

 Canines larger than the incisors. Molars with sharp cusps." 



Family, Didelphyidae : — American opossums, distributed from the 

 United States to Patagonia, arboreal in habit, usually carnivorous 

 or insectivorous .in diet. The limbs have five digits vifith claws, 

 the hallux is opposable. The tail is generally long and often 

 prehensile. The stomach is simple, the cEecum small. The 

 pouch is generally absent, but the young are often carried on 

 the back of the inother, their tails coiled round hers. Dentition, 

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4134' 

 Examples : — The Virginian or crab-eating opossum {Didelphys 

 marsupialis), with a pouch ; the woolly opossum (i?. lanigerd) ; 

 the aquatic Yapock (Chironectes) which feeds on fish and 

 smaller water animals. 

 Family Dasyuridse : — Carnivorous or insectivorous Marsupials. The 

 limbs have clawed digits, five in front, four or five behind. The 

 canines are generally large. The stomach is simple, there is no 

 CEecum. 

 Examples: — The Tasmanian wolf {Thylacinus), of dog-like form, 



dentition, , and the Dasyure {Dasyurus), civet-like, den- 



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tition, , are specialised as carnivores. The members of the / 



