20 MEGALONYX. 



The extent of the pulp cavity appears to be subject to some variation, probably 

 dependent upon the age of the animal. Thus, in the first superior molar contained 

 in the skull of Dr. Owen's collection, the pulp cavity extends about half the length 

 of the tooth, which measures nearly five inches ; but in the corresponding tooth in 

 the skull of Dr. Dickeson's collection, the pulp cavity reaches within three-fourths 

 of an inch of the triturating surface. The latter specimen, however, in its perfect 

 condition, has been at least an inch and a half shorter than the former, probably 

 from its having belonged to an older animal in which the tooth, having further 

 fulfilled its function, has been more worn away, for the alveolus is also more 

 shallow than that in the skull of Dr. Owen's collection. 



Characteristic of the family to which Megalonyx belongs, the teeth are entirely 

 destitute of enamel, and in transverse section they exhibit a central disk of very 

 porous dentine, surrounded by a much harder layer of the same substance, which is 

 also inclosed by a thinner layer of cementum. 



The harder dentinal layer is thickest where the teeth, in mastication, are most 

 subjected to attrition. It appears less dense than the corresponding substance in the 

 teeth of Mylodon, and the striae and punctse produced by the sides and extremities 

 of its tubuli are much more distinctly visible to the naked eye. It has also a con- 

 centric disposition, as exhibited by a number of fine lines. 



The cementum externally has a longitudinally striato-granulated appearance, and 

 in section also presents a concentric arrangement. 



In the first superior molars the harder dentinal layer is thickest externally and 

 antero-internally, and is thinnest postero-internally. The cementum also holds the 

 same relations of thickness in different positions. Upon the remaining molars the 

 harder dentine and cementum are thickest anteriorly and posteriorly, and are very 

 thin laterally. 



Cei-vical Vertebrce. — Of the vertebrae of the neck. Dr. Owen's collection contains 

 the atlas, axis, and three others, which appear to be the third, fifth, and seventh. 



The atlas (PI. VII, Figs. 1, 2) of Megalonyx Jeffersonii is about one-third less in 

 size than that of Mylodon rohushts, and possesses very nearly the same form. 



The upper arch of the atlas is convex transversely, and measures twenty lines 

 antero-posteriorly ; and it is surmounted by a conoidal protuberance. The lower 

 arch is nearly as convex as the former, but only measures an inch antero-posteriorly. 

 The transverse processes are broad, are slightly convex above, and below present a 

 deep concavity leading to the vertebral foramina. 



The spinal canal is vertically oval and measures two and a half inches in its 

 long diameter, and an inch and two thirds between the tubercles of attachment for 

 the transverse ligament. 



The articular facets for the occipital condyles are deeply concave and slope to 

 the spinal canal. The posterior articular facets are oblique, ovoid, and slightly 

 concave. The facet for articulation with the odontoid process of the axis is trans- 

 versely elliptical, and measures an inch and a half in its long diameter. 



The course of the vertebral artery is indicated by a tortuous passage, as follows : 

 A deep channel commences above at the back margin of the transverse process, 



