IO 



A Remarkable Ground Sloth 



Comparative formulas taken from Stock are : 



N othrotherium shastense 

 N othrotherium Y.P.M. specimen 

 Hafalofs longicefs 

 Hafalofs elongatus 

 Megatherium americanum 

 Myrmecofhaga jubata 



Hafalofs of the Santa Cruz formation is looked upon as the ancestor of N othro- 

 therium. As Stock says, the reduction of the number of thoracic vertebrae from twenty- 

 two to seventeen is the most fundamental change in the skeleton which has occurred, 

 and in this all Pleistocene ground sloths agree. I should add foot attitude, so clearly 

 indicated in our specimen, as another important change. 



The entire vertebral column of the Yale specimen agrees with Stock's descrip- 

 tion, except for the following details. 



Cervical Vertebrae. — The cervicals are relatively slenderer, a juvenile char- 

 acter. The spine of cervical 6 is relatively higher, being two-thirds that of the 7th but 

 slenderer. In Stock's description it is only slightly higher than the 5th; ours is half 

 again as high. 



CERVICAL MEASUREMENTS 



Atlas 

 Greatest transverse width 

 Greatest antero-posterior diameter 



4th cervical 

 Ventral length of centrum 



Width over anterior face of centrum 

 Greatest width across prezygapophyses 

 Height from ventral border of posterior face of 

 centrum to tip of neural spine 



Articulated cervicals 1 to 7 

 Length over all 



293.0 



ca. 280.0 



Thoracic Vertebrae. — The anterior thoracic possesses a large spine. With the 

 Yale specimen the epiphysis of this vertebra is one of the few preserved. Not only is 

 the spine widened antero-posteriorly in both the Rancho la Brea and the Yale speci- 

 mens, but in the latter the summit of the spine is broadened transversely as well. It 

 also differs in outline, especially posteriorly, from that described by Stock. 



