82 FIELD MUSEUM or NATURAL HISTORY ZOOLOGY, VOL. XIV. 



Considered as a series, the cervical vertebrae of Ccenolestes seem more 

 similar to those of Perameks than of any other marsupial. The imper- 

 forate atlas, the persistent cervical rib attached to the axis, the evenly 

 curved front edge of the neural canal of the axis, and the imperforate 

 seventh vertebra all are found in Perameles. The inferior lamellae of the 

 transverse processes of the third, fourth, and fifth cervicals are much 

 larger in Perameles than in Ccenolestes and this constitutes the principal 

 distinction between them. In comparison with those of other American 

 forms, the cervicals of Ccenolestes show a general resemblance to those of 

 Marmosa and Peramys and, among Australian small polyprotodonts, to 

 those of Phascologale , but the characters of the atlas and axis are dis- 

 tinctive. With the highly modified cervicals of Didelphis, comparison is 



scarcely- necessary. 







THORACIC VERTEBRAE. 



There are thirteen thoracic vertebrae as in all other marsupials with 

 the exception of Phascolarctos (n), certain species of Phascolomys (15), 

 and Notoryctes (i$). 1 The spine of the first thoracic is scarcely higher 

 than that of the last cervical (PL XII, fig. 4). The second spine is 

 slender and much the highest of the series, measuring about 3.5 mm. 

 The third to the eighth also are slender and progressively shorter and 

 more recumbent. The ninth is still shorter, and although directed 

 backward, it is longer basally and less acicular in form. The first 

 thoracic differs from the last cervical mainly in the elevation of the 

 transverse process and in its differentiation for articulation with the 

 rib. In most marsupials there is an abrupt transition from the last 

 cervical to the first thoracic, especially in the length of the spinous 

 process, but in Ccenolestes this break is between the first and second 

 thoracics. The only form examined which agrees with it in this respect 

 is Perameles nasuta. In Phascolarctos, the spine of the first thoracic only 

 slightly exceeds that of the last cervical but it is nevertheless the longest 

 of the thoracic spines and there is nowhere any marked break in the 

 series. Owen (1841) in his paper on the osteology of marsupials does not 

 mention a relatively short spine for the first thoracic of Perameles and 

 it is therefore possible that it is not found in all species of the genus. 



The tenth is the anticlinal vertebra and its spinous process is a low 

 upright plate with a straight crest occupying practically the entire length 



1 In a single specimen of Didelphis paraguayensis from Ceara, Brazil, there are 

 fourteen ribs instead of the thirteen normal in Didelphis. Moreover, there are 

 twenty vertebrae in the thoracico-lumbar series, the extra vertebra apparently 

 being the fourteenth, corresponding to the extra rib. Thfe is probably an abnor- 

 mality, but the examination of other specimens of this species is desirable, especially 

 since, as noted elsewhere (p. 86), it has other skeletal peculiarities. A pouch speci- 

 men of D. p. andinus has the normal thirteen ribs. 



