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THE AMERICAN NATURALIST. [Vol. XXXVII. 



and the nails have been modified into hooks by means of which 

 the body is kept in suspension. The number of digits is reduced 

 to two in Cholcepus and three in Brady pus. The carpal and 

 tarsal elements arc laterally compressed and there is some anas- 

 tomosis. This forms a more compact centre of resistance, while 

 the proximal bones develop a more or less complete ball and 

 socket joint in connection with the distal ends of the radius and 

 tibia, to permit a more perfect rotation. 



In the last sub-division (c) both the manus and pes have 

 become grasping organs. The hallux or pollex, or both, are 

 generally opposable. Many modifications occur in the pes very 

 similar to those already described for the marsupials. 



But in spite of these differences in main type there are 

 developed certain important characters which distinguish arbo- 

 real forms as a group from related terrestrial and aquatic types. 

 These like responses to the same conditions are to be observed 

 in what are otherwise most diverse forms. These characters are 



1. The tail is often prehensile and, as in some of the Cebidae, 



can move from branch to branch. Frequently correlated with 

 this adaptation is the loss of the thumb. 



2. Ectodermal spines are often developed. These may 

 occur on the root of the tail as in the Anomaluridae, on the 

 shoulder or on the feet as in (iymnura and some of the Anthro- 

 poidea. In all these cases the spines are climbing organs. 



3. The limbs are much elongated. This elongation may 

 occur in different segments in different forms. In the swinging 

 apes, it is the fore-arm rather than the hand which is elongated. 

 In the tree-sloths all the limb segments except the compressed 

 carpal ia and tarsal ia and proximal phalanges are lengthened, the 

 very long remaining phalanges and the claws forming a hook for 

 suspension. In other forms the tarsals are greatly lengthened 



obviously connec ted with the climbing and leaping habits of these 



4. The hallux or pollex, or both, are generally opposable. 

 This gives the hand or foot a stronger hold on the branches and 



