534 



CHORD AT A. 



The cervical vertebrae are nine in number, an exception 

 to the very general rule of seven in mammals. On the 

 other hand, the two two-toed species have seven and six 

 respectively. This anomaly may be probably connected 

 with the low organisation of the Edentata. The same varia- 

 tion is seen in the dorso-lumbar vertebrae. Our species has 

 usually nineteen to twenty, with fifteen to seventeen pairs of 

 ribs, but the two-toed species may have twenty-seven, with 

 twenty-four pairs of ribs. The neural spines all slope back- 

 wards and are not arranged about a centre of motion as in 

 the cursorial types. The pelvis fuses with at least six 

 vertebrae and the caudal vertebrae are vestigial. 



Fig. 368.— Lateral View of Skull of Three- 

 Toed Sloth. {Bradyptis tridactyhis. ) 



Note the peg-like molar teeth, the short muzzle and the forked 

 malar bone. 



It is in the limbs and limb-girdles that the arboreal 

 adaptation is most marked. The scapula is triangular, of 

 the climbing type. The coracoid process sometimes forms a 

 distinct bone, but is always large, and the clavicle is attached 

 to it. The arm-bones are very long and slender and the 

 radius and ulna are both present, the radius being capable of 

 rotation over the ulna (in supination and pronation). This 

 movement is usually developed in arboreal or even transition 

 arboreal types, as the variety of movement involved in such 



