THE MONODELPHIA. 281 



m. The MoNODELPHiA. — 111 the Monodelphia, the os 

 odontoideum very soon becomes anchylosed with the second 

 cervical vertebra, of which it appears merely as the odontoid 

 process ; and the cervical ribs early become inseparably united 

 with their vertebras. The coracoid is reduced to a mere pro- 

 cess of the scapula, and there is no epicoracoid similar to that 

 of the Ornithodelphia. 



Clavicles may be present or absent. When completely 

 developed they articulate directly, or by the intermediation of 

 more or less modified remains of the sternal end of the cora- 

 coid, with the sternum, and not with any interclavicle. The 

 acetabula are imperforate. The pelvis is devoid of marsupial 

 bones ; though, in some Garnivora, there are small cartilages 

 in the inner tendons of the external oblique muscle, which 

 have a corresponding form and relations. 



The anterior commissure and the corpus callosum, no less 

 than the cerebral "hemispheres themselves, vary greatly, the 

 brains of some Edentata very closely approaching those of the 

 Didelphia in respect of the corpus callosum and anterior com- 

 missure; while, as regards the hemispheres themselves, they 

 may either be so small as to allow the cerebellum to be com- 

 pletely exposed on the dorsal aspect, or so large as com- 

 pletely to cover it and project beyond it. The external sur- 

 face of the hemispheres, again, may be either perfectly smooth 

 or extremely convoluted. 



The cochlea is coiled spirally. The reproductive and 

 Urinary apertures, as a general rule, open quite separately from 

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

 testes may remain in the abdomen throi;ghout life, or may 

 pass into a scrotal pouch. But, when this scrotum forms a 

 distinct sac, it lies at the sides of, or beliind, the penis, and 

 not in front of it. The cystic urethra is always continuous 

 with that part of the urethra which travel ses the penis. 



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

 are fimbriated. The vagina is a single tulie, which may, how- 

 ever, be partially divided by a longitudinal partition. The 

 eremaster has no relation to the mammary 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 Monodelphia, as thus defined, 

 are divisible according to the characters of their placenta into 

 non-deciduata and deciduata. 



In the non-deciduala tlie foetal villi of the placenta are, al 



