534 CHORDATA. 
The cervical vertebree 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 Zdenfata.. The same varia- 
tion is seen in the dorso-lumbar vertebre. 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 
vertebrze and the caudal vertebre are vestigial. 
Fig. 368.—LATERAL VIEW OF SKULL OF THREE- 
ToED Story. (GBradypus tridactylus.) 
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 
