THE ORNITHODELPHIA. 275 



inner portions of the anterior margins of each pubis. Tliese 

 bones corresjDond with those which exist in a like position in 

 the Dldelp/iia^ and are called marsupial bones ; though the 

 term is peculiarly inappropriate, inasmuch as they have noth- 

 ing to do with the inarsupium^ or pouch, in w^hich the young 

 are sheltered in most of the Didelpliia. 



In the ujDper view of the brain the cerebellum is left com- 

 pletely uncovered by the cerebral hemispheres. The latter 

 are connected by only a very small corpus callosum. The an- 

 terior commissure, unlike that of any of the Bauropsida^ at- 

 tains a very great size, and the hippocamj^al sulcus is pro- 

 longed forward to the corpus callosum. 



In the internal ear, the cochlea is only slightly bent upon 

 itself, not coiled into a spiral, as in other Maminalia, The 

 stapes is imperforate and columelliform, and the malleus is very 

 large, while the incus is singularly small. 



There is a spacious cloaca common to the rectum, genital 

 and urinary organs, as in the Sauropsida and many Ichthy- 

 opsida. In both sexes a long urogenital canal opens into the 

 front part of the cloaca. At its anterior end there are five 

 distinct apertures — one in the middle line for the bladder, and 

 two on each side, which are the openings of the genital ducts 

 and of the ureters. Thus in these 3Iam7nalia, and in these 

 only, the ureters do not open into the lu'inary bladder. The 

 testes remain in the abdomen throughout life. The penis is 

 attached to the front wall of the cloaca, and is not united 

 directly with the ischia. It is traversed by a urethral canal, 

 which opens into the cloaca posteriori}', but is not directly con- 

 nected with either the seminal or the urinary passages. It is 

 probable that, during copulation, the posterior aperture of the 

 penial urethra is applied to the anterior aperture of the uro- 

 genital canal, so as to form a continuous passage for the semen. 



The ova of the female are very large and project from the 

 surface of the ovary, as in the jSaiiropsida. The mouths of 

 the Fallopian tubes are not fimbriated. There is no vagina 

 distinct from the urogenital chamber. The mammary glands 

 are situated, one upon each side of the middle line, in the 

 hinder part of the abdominal wall. The various ducts of the 

 gland open upon a small area of the integument which is not 

 raised up into a teat, so that, in the strict etymological sense 

 of the word, these animals are not Mco^imalia. The mam- 

 mary gland is compressed by the 2^^i^^nicidus eartiosus, and not 

 by any prolongation of the cremastei\ 



There is no sufficient evidence of the nature of the fcetai 



