368 WILD BEASTS OF THE WOULD 



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. 



