206 EDENTATES. 
ever, the presence of nine, instead of the usual seven, vertebree in the neck; that 
is to say, it is the tenth vertebra from the head which first carries ribs articulating 
with the breast-bone, although the ninth, and occasionally the eighth vertebra, is 
furnished with a pair of free floating ribs, which stop short of the breast-bone. 
There are several species of the genus characterised by differences in coloration, or 
by the relative length of the hair on the face; but the precise number of species is 

THE COMMON THREE-TOED SLOTH (4 nat. size). 
not yet definitely ascertained. In some kinds the coloration is uniform, while in 
others there is a dark stripe across the shoulders; the presence of the above- 
mentioned bright-coloured patch on the back cannot, however, as was once thought, 
be regarded as a specific character. The typical B. tridactylus inhabits the drier 
regions of the forests, while others, as noticed below, frequent the permanently 
flooded districts. As a rule, these animals appear to be silent; but it is recorded 
that a captive specimen, when dragged from the branch to which it was clinging, 
uttered a shrill scream. 
