EDENTATA. 



49 



Fig. 33. 



Fig. 34. 



Head of the Megatherium. 



sypus. The inferior maxillary bone varies 

 no less in its form in the different genera of 

 this incongruous order than the superior. It 

 is greatly elongated and very slender in the 

 Edentata proper, particularly in the Ant-eaters ; 

 the ascending plate is thin and small, the right 

 and left branches of the bone are united at the 

 symphysis to a considerable extent, and at a 

 very acute angle. In the Sloths this bone ex- 

 hibits a very different structure ; it is short and 

 deep, the ascending plate is broad and almost 

 square, the angular process is very large, and 

 the two branches of the jaw unite at the 

 symphysis without an angle, the anterior por- 

 tion of each side being curved inwards to meet 

 its fellow. In the Megatherium the body of 

 the bone is still higher and shorter, but the an- 

 terior part is prolonged into a narrow and de- 

 pressed groove somewhat similar to that of the 

 elephant. 



The vertebral column. The variation in the 

 form and construction of the vertebne will be 

 found to bear an exact relation to the habits of 

 the different genera. The cervical vertebra of 

 the A'i, Bradi/pus tridactylus, have always, until 

 very recently, been believed to form an excep- 

 tion to the general law, which assigns seven as 

 the strict number of these bones in the mam- 

 miferous animals. That this number should 

 exist equally in the hog and the giraffe is in- 

 deed a remarkable fact, and may be considered 

 as a striking illustration of the law by which 

 variations in volume in any particular system 

 of organs are provided for rather by the differ- 

 ence in volume or in the relative proportions of 

 the organs themselves, than by any abrupt 

 change in their number. The supposed excep- 

 tion to this law which now comes under our 

 notice consists in the fact that the neck of the 

 animal in question, (speaking of the part 

 rather in reference to its use than in strict ana- 

 tomical language,) is formed of nine vertebrae. 

 Two skeletons in my own possession, however, 

 have enabled me to demonstrate that the posterior 

 two of these vertebra? (Jig- 34) have attached to 

 them the rudiments of two pair of ribs in the 

 form of small elongated bones articulated to the 

 transverse processes of these bones, which are 

 therefore to be considered as truly dorsal ver- 

 tebrae, modified into a cervical form and func- 

 tion, suited to the peculiar wants of the animal. 

 The object of the increased number of ver- 

 tebrae in the neck is evidently to allow of a 

 more extensive rotation of the head ; for as 



VOL. II. 



Neck of the Sloth. 



each of the bones turns to a small extent upon 

 the succeeding one, it is clear that the decree 

 of rotation of the extreme point will be in pro- 

 portion to the number of moveable pieces in 

 the whole series. When the habits of this 

 extraordinary animal are considered, hanging 

 as it does from the under surface of boughs 

 with the back downwards, it is obvious that the 

 only means by which it could look downwards 

 towards the ground must be by rotation of the 

 neck ; and as it was necessary, in order to 

 effect this without diminishing the firmness of 

 the cervical portion of the vertebral column, to 

 add certain moveable points to the number 

 possessed by the rest of the class, the ad- 

 ditional motion was acquired by modifying 

 the two superior dorsal vertebrae, and giving 

 them the office of cervical, rather than in- 

 fringing on a rule which is thus preserved 

 entire without a single known exception. 



In the two-toed Sloth there is but one pair of 

 these rudimentary ribs, and consequently only 

 the first dorsal vertebra enters into the compo- 

 sition of the neck. 



The dorsal portion of the vertebral column is 

 particularly long in the Ant-eaters as well as the 

 Sloth. These vertebrae are also generally more 

 numerous in this than in most other groups the 

 great Ant-eater having sixteen, the Ai fourteen, 

 and the Unau no less than twenty-three a larger 

 number than is found in any other mammi- 

 ferous animal. The ribs offer some striking 

 peculiarities in their construction. In the Ant- 

 eaters and Armadillos they are excessively broad 

 with the exception of the first and second. In 

 the Myrmecophaga jubata and M. didactyla 

 they overlap each other in an imbricated man- 

 ner on the upper part, a conformation which 

 gives great solidity to the chest. The Sloths 

 and the Megatherium exhibit also considerable 

 breadth of the ribs, but to a much less extent 

 than that just described, and the latter animal, 



