THE MONODELPHIA. 329 



external siu-face of the hemisplieres again, may be eitlier 

 perfectly smootli or extremely convoluted. 



The cochlea is coiled spirally. The reproductive and 

 urinary apertures, as a general rale, open quite separately 

 from the rectum. The ureters always open into the bladder. 

 The testes may remain in the abdomen throughout life, 

 or may pass into a scrotal pouch. But, when this scrotum 

 forms a distinct sac, it lies at the sides of, or behind, the 

 penis, and not in front of it. The cystic lu'ethra is always 

 continuous with that pai-t of the lU'ethra which traverses 

 the penis. 



The ova are small, and the mouths of the Fallopian tubes 

 are fimbriated. The vagina is a single tube, which may 

 however be partially divided by a longitudinal partition. 

 The cremaster has no relation to the maimnary glands, 

 which are provided with distinct teats. 



The allantois is always well developed, and gives rise to a 

 placenta; and the young are born of large size, and 

 active. 



The great majority of the Monoddphia, as thus defined, 

 are divisible according to the characters of their placenta 

 into non-deciduata and deciduata. 



In the non-deciduata the festal villi of the placenta are, 

 at birth, simply withdrawn from the uterine fossse, into 

 which they are received, and no part of the maternal 

 substance is tkrown off in the form of decidua, or maternal 

 part of the placenta. In the deciduata, on the other hand, 

 the superficial layer of the mucous membrane of the uterus 

 undergoes a special modification, and unites, to a gi-eater 

 or less extent, with the villi developed from the chorion 

 of the fcetus ; and, at birth, this decidual and maternal 

 part of the placenta is thrown off along with the fcetus, 

 the mucous membrane of the uterus of the parent being 

 regenerated during, and after, each pregnancy. 



There are, however, two orders of existing monodelphoas 

 Mammalia, the nature of the placentation of which is not 

 yet fully made out. One of these is the Sirenia, the pla- 

 centation of which is unknown. The other is the ill-defined 



