2 68 SKETCH or THE HISTORY OF MAMMAMA. 



103. -Skeleton of Unau. 



destitute of the ligamentum teres, whence the head of 

 the thigh-bone is endowed with greater freedom of mo- 

 tion. In the sii (three-toed sloth) the neck consists of 

 nine vertebrae, instead of seven, the ordinary number in 

 mammalia, and the two tables of the skull in all the spe- 

 cies are separated by large air-cells, so that the small 

 bird-like brain is defended by a double case, a provision 

 against accidental falls, should the branch to which the 

 animal is clinging give way. 



Professor Owen observes, respecting the sloths, that 

 '' they illustrate the affinity or tendency to the oviparous 

 type, by the supernumerary cervical vertebrae, supporting 

 false ribs, and by the convolution of the windpipe in the 

 thorax, in the three-toed species ; by the lacertine 

 (lizard-like) character of three-and-twenty pairs of ribs 

 in the unau ; and by the low cerebral development, by 

 the great tenacity of life, and long-enduring irritability 

 of muscular fibre in both species." The muscles of the 



