MAY, 1921. AMERICAN MARSUPIAL, C^NOLESTES OSGOOD. 79 



atlas is illustrated in various stages from the wide open channel to the 

 heavy bony ring in a series of forms the sequence of which would be 

 about as follows: C&nolestes, Perameles, Didelphis, Phascologale, 

 Peramys, Dasyurus, Philander, Petaurus, Myrmecobius. In this series, 

 Didelphis is about midway between the extremes and various other 

 forms, including the macropods and phalangers, would occupy a similar 

 position. 



The atlas in Ccenolestes has no trace of a spinous process. The body 

 of the atlas is directed downward and slightly forward, being almost 

 vertical and nearly at right angles to the laminae of the arch. Its lower 

 part is thus a thin triangular plate with a hypapophysis or median 

 ridge on its posterior aspect and forming a pointed tubercle at its apex. 

 The odontoid process, therefore, does not have its usual flattened seat 

 but rests on the sharp edge of this nearly vertical body of the atlas. This 

 part of the atlas differs from that of practically all other marsupials, 

 although it is closely approached in Phascologale, Marmosa, and Peramys. 

 The neural canal of the atlas is slightly wider than deep and measures 

 4 mm. x 3.4 mm. 



The imperforate condition of the atlas in Ccenolestes, both anteriorly 

 and posteriorly, is doubtless primitive. It is found in the monotremes 

 and among living marsupials examined only in Marmosa, Peramys, and 

 Perameles. 1 It is characteristic also of the extinct Borhyaena but, so 

 far as known, of none of the other forms of the Santa Cruz beds. The 

 atlas, however, is unknown in all the fossil forms placed by Sinclair 

 (1906) in the family C&nolestidae. In other mammals, including all the 

 higher groups, no instances have been found in which both the atlanteal 

 and vertebrarterial foramina are absent. 



The axis has a high laminate spinous process similar in general to 

 that of the majority of marsupials. The anterior wing of this process 

 is not equal to the posterior and reaches only to the posterior part of the 

 dorsal surface of the atlas. The posterior wing overhangs the third 

 cervical. The front border of the neural arch seen from the side is a 

 continuous curve, and the prominent dentation found in Didelphis, 

 Peramys, Marmosa, Philander, Trichosurus, Petaurus, and some others 

 is absent. At the base of this curve is the prezygapophysis which is an 

 oval facet only very slightly elevated from the pedicel of the arch. The 

 odontoid process is completely ankylosed in the adult and forms a 

 simple projecting knob of the usual character. The body of the vertebra 

 is a fairly thick shield-shaped piece with two relatively large foramina 

 in the floor of the neural canal. The hypapophysis is peculiar. It^is 



l ln Notoryctes the atlas is imperforate, but this is obviously due to extreme 

 specialization and has no significance in this connection. (See Stirling, 1891.) 



