58 



them to the occipital region of the long and narrow head to have been very 

 considerable. 



In the quest of the affinities of these gigantic Sloth-like animals to their 

 diminutive existing congeners, a certitude as to the number of the cervical 

 vertebrae was especially desirable ; and the conditions under which those of the 

 Scelidothere have been discovered, — worked out of their stony matrix for the 

 most part by my own hands, — from the atlas to the first dorsal vertebra with its 

 broad rib, coupled with the evidence of the Mylodon, fully establishes our con- 

 fidence in the number of the cervical vertebrae exhibited in the skeleton at Ma- 

 drid. Nor is it less interesting to find, that although both the Mylodon and 

 Megatherium differ from the anomalous Ai in the number of their cervical ver- 

 tebrae, they both present precisely the same numbers of the remaining true ver- 

 tebrae as in the Ai, viz. 16 dorsal and 3 lumbar. The spine of the dentata in the 

 Megatherium is relatively lower and thicker than in the Mylodon, and is bifurcated 

 behind. It, however, partly overlaps the small pointed spine of the third cer- 

 vical, and those of the fifth, sixth and seventh progressively increase in length. 

 The atlas of the Megatherium is chiefly distinguished, in addition to its size, by 

 the tuberous rudiment of a spine on the upper arch, as in the Sloths, by the 

 more angular production of the broad or upper transverse process, and by the 

 strong tuberous rudiment of a lower transverse process at the posterior part of 

 the base of the preceding. The spinal canal is more contracted laterally : the 

 anterior articular cavities have a greater proportional vertical diameter, and are 

 more approximated. The grooves for the nerves at the posterior part of the 

 vertebra arc converted into canals by the extension of a bony bridge from the 

 neural arch to the upper margin of the posterior oblique processes. There are 

 two vascular foramina above and one below the root of the transverse process 

 in the Megatherium, as in the Mylodon. 



If the two-toed species of Sloth shows its nearer affinity to the extinct Me- 

 gatherioids in the normal number of cervical vertebrae, it deviates, on the 

 other hand, as much in the unusual extent of its dorsal or costal region, which 

 includes twenty-three, sometimes twenty-four vertebrae ; a greater number, as 

 Cuvier remarks, than exists in any other known mammalian quadruped. The 

 Ai has sixteen dorsal vertebrae, hke the Mylodon and Megatherium ; but ac- 

 cording to the view taken by Professor Bell of the nature of the two supernu- 



