CLASSIFICATION OF MARSUFPIALS. 745 
partition. Moreover, in Bennett’s kangaroo, the cecum 
opens independently into the sinus between the apertures 
of the distal portions of the vaginz, and forms the so-called 
third vagina. In Ferameles, 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 
vaginee 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- 
Fic. 404.—Lower jaw of kangaroo. 
a., Inflected angle ; 7., single incisox. 
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 
