328 A NEW MYLODON. 
curved, and the posterior corners are scarcely produced beyond the level of 
the articular facets, whereas in M. robustus they extend considerably beyond 
them. The positions of the vertebrarterial foramina are very 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 jeffersont. In our specimen, however, the posterior fora- 
men is 27 mm. behind the anterior and of very much smaller size. The trans- 
verse diameters are 18 mm. and 8 mm. respectively. The ventral and posterior 
aspects of the atlas aremuch asin 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 avis (Plate 4, fig. 20) differs from that of M. robustus 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 31mm. _ 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 five cervical vertebrae are present though without their trans- 
verse processes. The spines of the third and fourth cervicals are thick 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 spine next succeeding for about 
half its height. The result is that the neck vertebrae by thus firmly interlock- 
ing, have but little lateral play upon each other. The seventh cervical, as in 
M. robustus, has a concave articular facet for the capitulum 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. 
