152 WILD BEASTS OF THE WORLD 



larger than the rest; indeed, those in the upper jaw are considerably 

 smaller. One very curious point about these Three-toed Sloths is that 

 they are the only beasts which have more than the regulation number 

 of seven vertebrae in the neck for they have nine. This would seem 

 to be an admirable adaptation to allow the beast to twist its head about 

 as it hangs upside-down ; but then we are confronted with the awkward 

 fact that among the Two-toed Sloths the Brazilian species has the 

 ordinary seven neck-vertebrae, and Hoffmann's Sloth of Central America 

 only six the only exception to the rule of seven on the side of de- 

 ficiency. Cases like this show that anatomical characters are not 

 necessarily in themselves important, and are thus worthy of careful 

 note. 



There are several species of Three-toed Sloths, ranging, like the 

 Two-toed kinds, through tropical America ; they are often called by 

 the name of "Ai" said to be derived from their cry, the Brazilian 

 Two-toed species being known by the title of " Unau." In any case, 

 the note is not to be described as a "horrid scream," being rather a 

 bleat or a snort; but they are not noisy animals. 



