722 MAMMALIA. 



vagina; are apparently too narrow to allow for the passage of 

 the embryos. 



The allantois in Perameles, as already seen, forms a true allantoic 

 placenta ; in Phascolarctos it fuses with the subzonal membrane, 

 becomes highly vascular, and functions as an embryonic respiratory 

 organ, but does not unite with the uterine wall ; in all other Mar- 

 supials, so far as is known, it is small, only projects slightly into the 

 extra-embryonic body cavity, and is apparently functionless. Accord- 

 ing to Hill, the condition seen in Peravielcs is primitive, and the other 

 Marsupials show degeneration. The wall of the umbilical vesicle or 

 yolk-sac is highly vascular, and may unite with the uterine wall to form 

 a yolk-sac placenta. 



The gestation is short, only lasting a fortnight in the 

 opossum, about five weeks in the kangaroo ; whereas that of 

 the mare, for instance, is about eleven months. Except in 

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

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

 and here the delicate young are nurtured after birth. As 

 they are unable to suck, the milk is forced down their 

 throat, the mammary gland being compressed by the 

 cremaster muscle which covers it. Vague vestiges of a 

 marsupium are said to be visible in some Placentals. 



Classification of Marsupials. The Marsupials are divided 

 into two sub-orders, each of which contains four families. The two 

 sets are defined by the characters of the teeth, which are, of course, 

 adapted to habit. In the members of the first sub-order the incisors 



are numerous (not less than -), small, and almost equal in size ; while 



the canines are large, and the molars furnished with sharp cusps. The 

 whole dentition presents a striking resemblance to that of the Eutherian 

 Carnivores. To this group the name Polyprotodontia is applied, and 

 the forms included in it are typically carnivorous or insectivorous. The 

 cnecum is absent or very small. 



In the remaining families the incisors are usually 3 in number, and 



those above are of unequal size, the centre ones being largest. The 

 canines are usually small or absent ; the molars are furnished with 

 blunt tubercles, or transverse ridges. To these typically herbivorous 

 forms the name Diprotodont is applied ; they are more highly specialised 

 than the Polyprotodonts, and are more modern. They have a c?ecum. 



A. POI.YPROTODOXTIA. 



I. Family Didelphyida.'. American opossums, distributed from the 

 United States to Patagonia, arboreal in habit, usually carnivorous 



