METATHERIA, DIDELPHIA, OR MARSUPIALTA. 721 



marsupial bones in front of the pubic symphysis. These 

 have no connection with the marsupium, as is evident from 

 the fact that they occur in both sexes ; they are sesamoids 

 developed in the inner tendon of the external oblique 

 muscle of the abdomen. 



The teeth cannot be readily reduced to the typical Eutherian formula. 

 According to recent research, the milk set is degenerate, and is usually 

 represented only by the last premolar, which in most cases cuts the 

 gum, and is for a time functional. The other teeth correspond to the 

 permanent set of the Eutheria. According to another view, the 

 functional teeth are milk-teeth. 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 



FIG. 355. Lower jaw of kangaroo. 

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



urogenital aperture, and in the majority of cases the 

 anus lies so much within the urogenital sinus that the 

 arrangement may be described as cloacal. The scrotal sac 

 containing the testes lies in front of the penis a unique 

 position. 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 some- 

 times 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 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 

 46 



