CLASSIFICATION OF MARSUPIALS. 745 



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

 opens independently into the sinus between the apertures 

 of the distal portions of the vaginae, and forms the so-called 

 third vagina. In Perameles, although such a median 

 passage does not exist "in the young female, it is formed by 

 a process of rupture at the period of parturition. The true 

 vaginae are apparently too narrow 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- 



FIG. 404. Lower jaw of kangaroo, 

 a. t Inflected angle ; /., single incisor. 



ing to Hill, the condition seen in Perameles 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 tc 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 



