188 SKELETON OF THE SLOTH. 



most constant, and forms tlie single-hoofed fore-foot of 

 the horse. 



The scapula, 51, in the fore-limb repeats or answers to 

 the ilium, 62, in the hind-limb ; the coracoid, 52, to the 

 ischium, 63; the clavicle, 58, to the pubis, 64; the hu- 

 merus, 53, to the femur, 65 ; the radius, 55, to the tibia, 

 66 ; the ulna, 54, to the fibula, 67 ; the carpus, 56, repeats 

 the tarsus, 68 ; and the metacarpus and phalanges of the 

 fore-foot repeat the metatarsus and phalanges of the hind- 

 foot : they are technically called " serial homologues," or 

 " homotypes," and each bone in the carpus can be shown 

 to have its homotype in the tarsus. (See Archety2')e of the 

 Vertehrate SMeton,'' p. 167.) 



SKELETON OF THE SLOTH. 



The transition from the quadrupeds with hoofs to those 

 with claws seems in the present series to be abrupt; but 

 it was made gradual by a group of animals, now extinct, 

 which combined hoofs and claws in the same foot. Some 

 of the outer toes, at least, were stunted and buried in a 

 thick callosit}^, for the ordinary purpose of walking, 

 whilst the other toes were provided with very long and 

 strong claws for uprooting or tearing off the branches of 

 trees. These singular beasts were of great bulk, and ap- 

 pear to have been peculiar to America. As restored by 

 anatomical science, they have received the names of Me- 

 gatJierinm, Megalonyx^ Mylodon^ &c. They were huge ter- 

 restrial sloths; the present remnants of the family consist 

 of very few species enabled by their restricted bulk to 

 climb the trees in quest of their leafy food, and peculiarly 

 organized for arboreal life. The toes, which were modi- 



