324 THE ANATOMY OP VEETEBRATED ANIMALS. 



tlie Monodelphia in possessing either bones, or cartilages, 

 attached to the pubes, in the position of the so-called 

 niarsnpial bones of the Omitliodelphia. 



Again, the brain, the cerebral hemispheres of which may 

 or may not have a convoluted surface, is provided with a 

 very small corpus callosum, and a large anterior com- 

 missui-e. The hippocampal sulcus is prolonged forwards 

 over the corpus callosum. 



The crura of the corpus cavemosum of the penis are not 

 fixed to the ischium. 



The embryo does not become connected with the parent by 

 villi developed from the aUantois, and it is bom in a very 

 imperfect condition. 



Certain characters are peculiar to the Didelphia. Thus, 

 the testes of the male pass into a scrotum, which is sus- 

 jpended in front of the penis. In the female, the cremaster 

 muscle is largely developed, and spreads over the surface of 

 the mammary gland, which it compresses, so as to drive 

 the milk out of the projecting teat. There is no fossa 

 ovalis on the right side of the septum of the auricles. 

 Very generally, though not invariably, the Bidelphia possess 

 what is termed a marsupial pouch, which is a sort of bag, 

 foi-med by a fold of the integument of the abdomen, into 

 which muscular fibres of the pianniculus carnosus extend. 

 These suj)port the ventral wall of the pouch, and are 

 capable of closing its mouth, which may be directed either 

 forwards or backwards. The mammary glands lie in the 

 dorsal wall of this pouch, into which the teats project. 



There is no direct communication between the female 

 generative organs and the pouch ; but the minute young 

 are transported, in the blind and imperfect state in which 

 they are bom, into the interior of the marsupium, and each 

 becomes attached to a nipple, which exactly fills its mouth. 

 To this it remains attached for a considerable period, the 

 milk being forced down its throat by the contraction of the 

 cremaster muscle. The danger of suffocation is averted 

 by the elongated and conical form of the upper extremity 

 of the larynx, which is embraced by the soft palate, as in 



