86 ZOOLOGY OF THE VOYAGE OF THE BEAGLE. 



With respect to the At, or three-toed Sloth, " an animal, great part of whose life, 

 when not engaged in eating, is spent in sleeping on trees,— an easy attitude for repose 

 is most essential to its comfortable existence ; and accordingly we find, that the 

 auxiliary vertebrae at the base of the neck contribute to produce that flexibility of 

 this organ which allows the head of the animal to incline forwards and rest upon 

 its bosom." Dr. Buckland, from whose Paper on the " Adaptation of the Struc- 

 ture of the Sloths to their peculiar Mode of Life,"* the preceding judicious 

 physiological remark is quoted, adduces the authority of Mr. Burchell in proof 

 that the Sloth can in a remarkable manner and with great facility twist its head 

 quite round, and look in the face of a person standing directly behind it, while 

 at the same time the body and limbs remain unmoved. A single glance at the 

 length and slenderness of the cervical region of the spine, and of the feeble condi- 

 tion of the transverse and spinous processes in the vertebrae composing that part of 

 the skeleton of the Sloth, is enough to show its adaptation to increase the rotatory 



motion and flexibility of the neck. 



In describing the skeleton of a species of Armadillo (JDasypus 6-cinctus, Linn.)| 

 I was led in like manner to point out the subserviency of the peculiarities of the 

 cervical vertebrae to the habits and mode of life of that animal ; observing that the 

 " anchylosis of the cervical vertebrae obtains in the Cetacea, as well as in the genus 

 Dasypus, and that as in the aquatic order this firm connexion of the cervical ver- 

 tebrae assists materially in enabling the head to overcome the resistance of the 

 dense fluid through which they perpetually move, so in the Armadillos a like 

 advantage may be derived from this structure during the act of displacing the 

 denser material in which they excavate their retreats. "J 



Having in view these well-marked examples of the subserviency of the struc- 

 ture of the bones of the neck to the habits of existing species of the order Bruta, 

 I proceeded to investigate the structure of the corresponding part of the skeleton 

 in the Scelidolherium, hoping thereby to gain a new and useful element in the 

 determination of the problem at present under discussion, as to the affinities and 

 habits of the extinct Megatherioid quadrupeds. 



The fossil, in its original state, yielded a view of so much of the anterior part 

 of the bodies of the cervical vertebrae as proved that they were neither so nume- 

 rous as in the Sloth, nor anchylosed together as in the Armadillos : after a long 

 and careful chiselling at the hard matrix in which they were imbedded, the trans- 



* Linn. Trans, vol. xvii. (1833) p. 17. t Zool. Proceedings, 1832, p. 134. 



X The anterior prolongation of the sternum in front of the neck and the corresponding anterior position of 

 the clavicles and scapulae occasions a transference of such a proportion of the moving powers of the head from 

 the cervical vertebrae to these bones in the mole, as renders any modifications of these vertebrae, like those in the 

 Armadillo, uncalled for. 



