594 LAND MAMMALS IN THE WESTERN HEMISPHERE 



peculiarities of the order. The neck in the tree-sloths has 

 eight or nine vertebrae, the only instances known among mam- 

 mals in which the normal number of seven is departed from. 

 In the armadillos and fglyptodonts several of the neck- vertebrae 

 are coossified into a single piece, but the atlas is always free, 

 so as to permit the movements of the head. In the posterior 

 part of the dorsal and in the lumbar region the articulations 

 between the successive vertebrae are by far the most complex 

 and intricate known among mammals ; in the tree-sloths 

 these have degenerated, though still plainly indicated. In the 

 fglyptodonts, which were covered with a huge, tortoise-like 

 carapace, mobility of the backbone was needless, and so all of 

 the dorsal vertebrae were united into one long piece and the 

 lumbars were coossified with one another and with the sacrum. 

 The sacrum consists, throughout the order, of a very large 

 number of vertebrae and is attached to the hip-bones at two 

 different points, instead of only one, as in other mammals. 

 The tail varies much in length and thickness ; in the tree- 

 sloths it is extremely short and in the anteaters very long and 

 bushy, prehensile in the arboreal members of the group ; in 

 the fground-sloths, especially the gigantic forms, it was of 

 immense thickness ; while in most of the fglyptodonts a vary- 

 ing number of the terminal vertebrae were fused together. The 

 sternal ribs are better developed than in any other mammals, 

 and in the anteaters and fground-sloths they articulate with 

 the breast-bone by regular synovial joints, and each rib has 

 head and tubercle like a vertebral rib. 



In the limbs and feet there is great variety, according to the 

 manner of their employment. The shoulder-blade has a very 

 long acromion and very large coracoid, which long remains 

 separate from the scapula; collar-bones are very generally 

 present, though often in much reduced condition. The hip- 

 bones have in the tree-sloths, fground-sloths and fglypto- 

 donts a much expanded anterior element, which in the other 

 groups is narrow. The humerus usually has very prominent 



