1887.] NATURAL SCIENCES OF PHILADELPHIA. 67 



in arboreal creatures. In the sloth ( Cholcepus) flexion is absent, 

 the limbs being advanced by a SAvinging motion at the shoulder and 

 the hip. 



The unaided eye receives the impression of backward movement 

 but fails to be impressed with forward movement. It may hence be 

 inferred that the former is a quicker movement than the latter. 



In the fore limb the last state of extension of the forearm answers 

 to the action of the extensors of the carpus and of the digits. In the 

 less delicate movements of the hind limb the muscles which extend 

 the tarsus and the toes move the foot with less precision and it is 

 likely with less speed. 



Biological Department of the University of Pennsylvania, March 1st, 1887. 



