THE DIDELPHIA. 277 



quen Jy are imperforate in the dry skeleton. The cochlea i3 

 coiled upon itself. 



There is a shallow cloaca, the sphincter muscle being com- 

 mon to the urinary and genital apertures, but there is no such 

 urogenital chamber as in the Monotremata. The ureters open 

 directly into the bladder. 



In the male, the urogenital part of the urethra, and that 

 >\ hich traverses the penis, form one continuous canal, which 

 opens outward only at the extremity of the penis. 



Ill the female, the vaginal is perfectly distinct from the 

 urinary passage. The mouths of the Fallopian tubes are fim- 

 briated, and the ova are not larger than those of the Mono- 

 delphia. 



The mammary glands are provided with long teats. 



In all the preceding characters the Didelphia agree with 

 the Moyiodelphia, and differ from the Ornithodelphia. 



But they agree with the Ornithodelphia, and differ from 

 the Monodelphla in possessing either bones or cartilages, at- 

 tached to the |)ubes, in the position of the so-called marsupial 

 bones of the Ornithodelphia. 



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 commissure. The 

 hippocampal sulcus is prolonged forward over the corpus cal- 

 losum. 



The crura of the corpus cavernosum of the penis are not 

 fixed to the ischium. 



The embryo does not become connected with the parent 

 by villi developed from the allantois, and it is born in a very 

 imperfect condition. 



Certain characters are peculiar to the Didelphia. Thus, 

 the testes of the male pass into a scrotum, which is suspended 

 in front of the penis. In the female, the cremaster muscle is 

 largely developed, and spreads over the surface of the mam- 

 mary gland, whicb 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 Didelphia possess what is termed a marsu- 

 pial pouch, which is a sort of bag, formed by a fold of the in- 

 (egument of the abdomen, into which muscular fibres of the 

 panniculus carnosus extend. These support the ventral wall 

 of the pouch, and are capable of closing its mouth, which may 

 be directed either forward or backward. The mammary glanda 

 ie in the dorsal wall of this pouch, into which the teats project 



