78 FIELD MUSEUM OF NATURAL HISTORY ZOOLOGY, VOL. XIV. 



laminae which are weak as compared to the bodies. They are well 

 separated from each other and have no tendency to imbrication. All 

 except the first and seventh have vertebrarterial foramina. Only the 

 first and second have hypapophyses. 



The atlas is practically entire, having a body which evidently 

 ossifies in normal manner and having no large foramina for the passage 

 of nerves and blood vessels. A very minute perforation (PI. XI, fig. i, 

 n. /.) at the posterior dorsal base of the transverse process is of doubtful 

 significance, but its position does not seem to indicate homology with 

 either of the usual foramina of the atlas. A perforation with exactly 

 similar relations is found in the extinct Borhyaena (Sinclair, 1906, p. 350, 

 PI. LIII). The cephalic border of the neural arch of the atlas is nearly 

 straight for about the width of the neural canal and thence on either 

 side it is angled latero-caudad to meet the base of the cephalic articular 

 process. A groove between this angle of the border of the arch and the 

 articular process serves for the passage of the first cranial nerve and 

 supplies the office of the foramen found in most other marsupials. 

 That this foramen is formed by the closing of the points bounding the 

 mouth of this notch is shown by immature examples of Didelphis in 

 which the union is slightly cartilaginous. In the adult of Didelphis, the 

 foramen is well isolated as it is in Philander, Dasyurus, Myrmecobius, 

 Thylacinus, Sarcophilus, Phascologale, Trichosurus, Petaurus, Petrogale, 

 Macropus, Phascolomys, and Phascolarctos. The only living marsupial 

 genera agreeing with Canolestes in this open "intervertebral notch" are 

 Marmosa, Peramys, and Perameles. The atlas of Perameles is otherwise 

 similar to that of C&nolestes, but that in Marmosa and Peramys resembles 

 it still more closely, differing chiefly in the pedunculation of the trans- 

 verse process and its slight obliquity and in the tendency to the forma- 

 tion of a vertebrarterial canal with a partly cartilaginous border. 



The transverse process of the atlas has decided obliquity to the spinal 

 axis and is relatively short and broad with no constriction at its base and 

 with only a shallow groove between it and the posterior zygapophysis 

 in the position corresponding to the usual course of the vertebrarterial 

 canal. The only genera examined in which the arterial canal has com- 

 plete bony boundaries are Philander, Dasyurus, Sarcophilus, Myr- 

 mecobius, and Petaurus. In all others except Canolestes, however, the 

 transverse process is pedunculate and in some, as Phascologale and 

 Peramys (Marmosa and Didelphis to a lesser extent), there is a slight 

 bony elevation on the pedicel of the arch cephalad or laterad of the 

 postzygapophysis from Which a cartilage extends to the posterior angle 

 of the transverse process and forms the arterial canal. So far as material 

 at hand shows, the gradual formation of the vertebrarterial canal ofjthe 



