74 



a similar modification affects the claw-core, the narrow basal part of which is divided by 

 sharp borders from the sides. The point of the core projects about 3 inches in advance 

 of the lower border of the sheath: its form, broken in the specimen, is restored in 

 dotted outline in the figures. 



The distal articular surface of the fourth metatarsal shows that it supported a phalanx 

 so articulated with it as to have no movements of flexion or extension, and only a slight 

 degree of bending from side to side. The illustrations of the Madrid skeleton, especially 

 plate 5, fig. 5, show that this phalanx (Plate XXVI. fig. 1, iv, i) was very short, and that 

 it supported a second phalanx (ib. iv, s) of a subhemispheric form, terminated obtusely. 



The distal articular surface of the fifth metatarsal indicates that it supported a pha- 

 lanx smaller than the proximal one of the fourth toe ; and the figure, above cited, of the 

 Madrid skeleton shows such a phalanx (ib. v, i), and also a second small stunted hemi- 

 spheric phalanx (ib. v, i)* ; and this, from the analogy of the Mylodon, is most probably 

 the true structure. 



If we contemplate the bones of this singularly constructed foot in their natural 

 co-adaptation, the same relation of the osseous masonry to the transference of pressure 

 from the leg to the outer border of the foot will be appreciated, as has been pointed out 

 in the fore limb. The broad surface of the astragalus — the great keystone of the tarso- 

 metatarsal arch (Plate XXVI. fig. 1, a) — transmits the superincumbent weight in two 

 chief directions, backward upon the massive heel-bone (b), forward upon the metatarsus. 

 By the naviculare (c) it is transmitted through the ectocuneiforme (e) and the produced 

 outer angle of the base of the mid-metatarsal (m s) to the fourth (m <), and thence to 

 the fifth metatarsal (m t). The cuboides (d), receiving the weight from both astragalus 

 and naviculare, transmits it by its produced fore part to the base of the fourth meta- 

 tarsal ; and partly by that medium, but chiefly by direct articulation, to the side of the 



A 



base of the fifth metatarsal. The tendency in the cuboides to yield under this pressure 

 and slip back, is resisted by the abutment of the calcaneum (b, t) against its back part. 



§ 9. Comparison of the Bones of the Hind Foot. 



No known recent Mammal offers in its astragalus any repetition of the peculiarities 

 of that bone in the Megatherium. In the Anteaters and Armadillos the upper surface 

 of the astragalus has the usual configuration, and is received into a deep tibio-peroneal 

 mortice. In the Sloths the outer part of the astragalus is excavated by a deep cell, in 

 which the pivot-shaped end of the fibula rotates: the inner side is applied, as usual, 



• In the original memoir, by Beu and Gakeiga, on the Skeleton of the Megatherium at Madrid, the 

 metatarsus and toes of the hind foot are said to agree with those of the fore foot, except that there is only 

 one toe with a claw, instead of three (" Tambien se advierte que en este hay solo un Dedo con una, quando 

 en la Mono Be registran tres ; en lo demas convienen en un todo," p. 16). In the description of the outer- 

 most (fourth developed) toe of the fore foot it is stated that it has two phalanges, and that there is 

 nothing to be remarked except that they are rounded (" Los dos Falanget del quarto que se reconocen en 

 J. y L. no tienen cosa que advertir mas que son casi redondas," p. 13). 



