SKELETON OF BRUT A. 413 



astragalus. In Brad, tridactylus the calcaneum is remarkably 

 long and compressed. The scaphoid, cuboid, and cuneiform bones 

 have become confluent with each other and the metatarsals, of 

 which the first, i, and fifth, u^ exist only in rudiment. The other 

 three have likewise coalesced with the proximal phalanges of the 

 toes which they support. In the Brad. didactyJus the ento- and 

 meso-cuneiform bones, the rudimental metatarsal of the hallux, 

 and the metatarsal of the second toe are confluent into one bone : 

 the rudimental metatarsal of the fifth toe has not become united 

 with that of the fourth toe. The functional toes have long pre- 

 hensile claws like those of the fingers ; by the peculiar ankle- 

 joint the foot is turned inward, and the advantage in grasping is 

 obtained at the cost of the power of stepping on flat ground. 



The Megathere has a pivoted articulation of the foot with the 

 leg, but the process and the cavity are on reverse parts of the 

 ankle-joint, and the astragalus sends a process to fit a cavity in 

 the tibia. The result, in the inflection of the hind foot, is nearly 

 the same : but an enormous calcaneum, fig. 279, cl, and metatarsal 

 of the fifth toe, ib. v, rest broadly on the ground. The lower 

 surface of the astragalus transmits the superincumbent weight in 

 two directions ; backward upon the heel-bone and forward upon 

 the metatarse. By the naviculare it is transmitted through the 

 ectocuneiforme and the produced angle of the base of the mid- 

 metatarsal to the fourth and thence to the fifth metatarsal. The 

 cuboides receivino; the weigfht from both astras^alus and naviculare 

 transmits it by its produced fore-part to the base of the fourth 

 metatarsal, and partly by that medium, but chiefly by direct 

 articulation, to the side of the fifth metatarsal. The tendency of 

 the cuboides to yield under this pressure and slip back is resisted 

 by the abutment of the calcaneum against its back part. The 

 digitus medius, ib. Hi, was alone developed to sustain and wield 

 a claw ; but this was of enormous size, and must have had the 

 power of a pick when worked by the lever of the long heel-bone. 

 The first and second toes were not present, nor was the ento- 

 cuneiform bone. The two outer toes, iv and v, terminated in 

 tuberous phalanges, evidently imbedded, in the living animal, 

 in a hoof-like thickening of the outer border of the foot. The 

 outer side of the fore-foot presents a similar modification for quasi- 

 ungulate progression on the ground. Thus the Megathere, My- 

 lodon, and allied great terrestrial Sloths seem to have combined 

 ungulate and unguiculate characteristics in the same extremity. 



The principle of viewing structures and instruments, in reference 

 to the work that they may do, is shown to be good in gaining in- 



