THE ORNITHODELPHIA. 321 



The central portion of the acetabulum remains unossified, 

 and hence, in the dry skeleton, appears perforated, as in 

 Aves, Ornitlwscelida, and Crocodilia. 



The inner tendons of the external oblique muscles are 

 ossified for a considerable distance ; and these ossifications 

 appear in the dry skeleton as bones, which are articulated 

 with the inner portions of the anterior margins of each 

 pubis. These bones correspond with those which exist in 

 a like position in the Videlphia, and are called marsupial 

 bones; though the term is peculiarly inappropriate, inas- 

 much as they have nothing to do with the marsupium, 

 or pouch, in which the yoiuig are sheltered in most of the 

 DidelpTiia. 



In the upper view of the brain the cerebellum is left 

 completely uncovered by the cerebral hemispheres. The 

 latter are connected by only a very small corpus callosum. 

 The anterior commissure, unlike that of any of the Saurop- 

 sida, attains a very great size, and the hippocampal sulcus 

 is prolonged forwards to the corpus callosum. 



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

 itself, not coiled into a spiral, as in other Mammalia. The 

 stapes is imperforate and columelliform, and the malleus is 

 very large, while the incus is singularly small. 



There is a spacious cloaca coromon 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 Mam- 

 malia, and in thece only, the ureters do not open into the 

 urinary bladder. The testes remain in the abdomen through- 

 out life. The penis is attached to the front wall of the 

 cloaca, and is not united directly with the ischia. It is 

 traversed by an urethral canal, which opens into the cloaca 

 posteriorly, but is not directly connected with either the 

 seminal or the urinary passages. It is probable that during 

 copulation, the posterior aperture of the penial urethra is 



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