CHARACTERS OF MARSUPIALS. 685 



The functional teeth are those of the milk set, with the 

 probable exception of the third (or rather fourth) premolar. 

 It may be, however, that this successional premolar is a 

 retarded milk tooth, intermediate in position between pm. 2 

 and pm. 3. In living Marsupials, there seems to be a 

 suppression of what, in typical placentals, would be called 

 the second premolar. 



A common sphincter muscle surrounds the anus and the 

 urinogenital aperture, and in the females (except kangaroos) 

 the anus lies so much within the urinogenital sinus that the 

 arrangement maybe described as cloacal. The scrotal sac 

 containing the testes lies in front of the penis. The genital 

 ducts of the females are often separate throughout, so that 

 there are two uteri and two vaginae. But the bent proximal 

 parts of the vaginae sometimes fuse and form a caecum, 

 which, according to the degree of fusion, may be a single 

 tube or divided by a partition. Moreover, in Bennett's 

 kangaroo, the caecum opens independently into the cloaca 

 between the apertures of the distal portions of the vaginae. 



The allantois may be small, and may fail to reach the 

 subzonal membrane. In no case is there an allantoic 

 placenta. The yolk-sac is large and adheres to a portion 

 of the subzonal membrane. From this region in some 

 cases, e.g., opossums, non-vascular villi are given off, which 

 enter into close connection with the glands of the uterine 

 wall. The embryo is also attached to the uterus by 

 amoeboid processes from the subzonal membrane. 



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. 



Families of Marsupials. 



A. POLYPROTODONTIA. " Incisors numerous, small, subequal 

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



