686 



MAMMALIA. 



saved and insulated, the Marsupials, have evolved in many 

 different directions. 



The brain is less developed than in Eutherian Mammals, 

 for the convolutions are simple or absent, the anterior com- 

 missure is large, the corpus callosum is practically absent. 

 In the skeleton there are several peculiarities ; thus the 

 angle of the lower jaw is more or less inflected, except in 

 the genus Tarsipes ; the jugal reaches far back to share in 

 making the glenoid cavity ; there is practically only one set 

 of teeth ; there are more incisors above than below (except 

 in the wombat), and the number of incisors sometimes 

 exceeds three on each side. There are usually epipubic or 

 marsupial bones in front of the pubic symphysis. These 

 have no connection with the marsupium, as is evident from 



Fig. 301. — Lower jaw of kangaroo. 

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



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 

 urogenital aperture, and in the females (except kangaroos) 

 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 



