90 ON CLASSIFICATION. 



males are lodged in a scrotum, which is suspended in front of 

 the penis ; and the vasa deferentia open into a complete and 

 continuous urethra, which is also the passage by which the urine 

 escapes from the bladder, and is perfectly distinct from the pas- 

 sage for the faeces, though the anus and the termination of the 

 urethro-sexual canal are embraced by the same sphincter. 



The corpus callosum is comparatively small, as in the Orni- 

 thodelpliia. 



It is stated that the allantois of the embryo is arrested in its 

 development, and gives rise to no placenta. The umbilical sac 

 is said to acquire a large proportional size ; but whether it plays 

 the part of a placenta for the short period of intra-uterine life, 

 or not, is unknown. 



The young are born of very small size, and in a singularly 

 imperfect condition ; but being transferred to the marsupium, 

 and becoming attached to a long nipple, they are supplied with 

 milk until they are able to provide for themselves — the milk 

 being, at first, forced into their mouths by the action of a muscle 

 spread over the mammary gland. 



In the Monodelphia, the angle of the lower jaw is not 

 inflected, and they may or may not be provided with teeth. 

 They never possess " marsupial " bones. The uterine dilatation 

 of the oviducts is always considerable, and whether they have 

 common or distinct apertures, the vagina is a single tube, 

 though it may be partially divided by a septum. The testes 

 may vary much in position ; but, if they are lodged in a scrotal 

 pouch, it is never pendulous by a narrow neck in front of the 

 penis, as in the Didelphia. 



The urinary bladder opens into a distinct urethra, which, 

 directly or indirectly, receives the vasa deferentia in the male. 



The corpus callosum is very variable in its development, 

 commonly attaining a much larger size than in the preceding 

 groups ; the optic lobes are divided into four portions. 



The young are nourished within the uterus until such time 

 as they are competent to suck milk from the teats of the parent, 

 to which end the chorion always develops processes or villi, 

 which are well supplied with vessels brought to them by the 

 allantois. These processes becoming interlaced more or less 



