328 A NEW MYLODON. 



curved, and the posterior corners are scarcely jiroducetl beyond the level of 

 the articular facets, whereas in M. robusius they extend considerably beyond 

 them. The positions of the vertebrarterial foramina are \'eiy different. In 

 Owen's species the anterior and posterior foramina of the dorsal side are so 

 close together as to be nearly in the same depression, a condition which is fully 

 realized in Megalonyx jeffersoni. In our specimen, however, the posterior fora- 

 men is 27 mm. behind the anterior and of very mucli smaller size. The trans- 

 verse diameters are 18 mm. and 8 mm. respectively. The ventral and posterior 

 aspects of the atlas are much as in M. robustus. It measures: — greatest trans- 

 verse diameter, 193; greatest depth 106; breadth across anterior articulating 

 facets, 117; breadth across posterior articulating facets, 89; least longitudinal 

 diameter in midventral line, 37. 



The axis (Plate 4, fig. 20) differs from that of M. robusius mainly in that the 

 superior margin of the spinous process is at a much less angle to the long axis, 

 due to greater elevation of its anterior projection. The articular facet of the 

 odontoid process faces more ventrally and the cranial articular facets are appar- 

 ently more elliptical, with diameters 49 X 31 nun. Posteriorly the dorsal spine 

 is deeply hollowed out to receive the spinous process of the third cervical. The 

 height from the ventral border of the centrum to the tip of the spine is 130 mm.; 

 the diameters of the posterior face of the centrum are: — transverse, 54; vertical, 

 41. 



The remaining ^I'c cervical vertebrae are present thougli without their trans- 

 verse processes. The spines of the thiril and fourth cervicals are tliick and 

 rugose, but become more compressed and smooth on those behind. The cranial 

 articular processes become also broader and more roughened on their anterior 

 surfaces. The front edge of the neural spine of the third vertebra is produced 

 in a ridge forward, that fits into a deep hollow in the back of the spine of the 

 axis. In the fourth cervical this ridge does not bow forward but slopes evenly 

 to the tip of the spine, and the same is true of the remaining cervicals. Begin- 

 ning with the third, the posterior margins of the neural spines are produced 

 backward so as to enfold the anterior edge of the si)ine next succeeding for about 

 half its height. The result is that tlie neck vertebrae by thus firmly interlock- 

 ing, have but little lateral play ujion each other. The seventh cervical, as in 

 M. robustus, has a concave articular facet for the cajMtulum of the first rib, 

 situated at each side of the centrum posteriorly. The anteroposterior length 

 of the centrum is about the same (34 mm.) in the third to fifth cervicals but 

 is slightly greater (38) in the two others. 



