286 THE ANATOMY OF VERTEBRATED ANIMALS. 



astragalus looks almost directly outward, and the pivot of the 

 fibula is more nearly horizontal, when the leg is vertical. The 

 tibial facet of the astragalus locks directly upward. Hence, 

 the movement of the pes is more exclusively one of flexion 

 and extension than in the Ai. No anchylosis of the tarsal, 

 metatarsal, and phalangeal bones occurs, but the rotation of 

 the distal moiety of the tarsus ujDon the astragalus is much 

 more complete and permanent than in the Ai. The calcane^ 

 um is twisted round under the astragalus, in such a manner 

 that its proper external face becomes inferior, while the articu- 

 lar surface for the cuboid is not only below, but is partially 

 internal to, the navicular facet of the astragalus. As a result 

 of this position of the cuboid, the outer metatarsals, which it 

 supports, are placed directly beneath the inner ones, and the 

 pes rests absolutely upon its outer edge, the plane of the sole 

 beino: vertical. 



The Sloths, it thus appears, are naturally club-footed ; but 

 neither in the Ai, nor in the Unau, does this depend in any 

 way on the structure of the ankle-joint. On the contrary, it 

 results, in the Unau, from the manner in which the calcaneiim 

 and naviculare articulate with the astragalus ; and, in the Ai, 

 from the action of the muscles on the tarso-phalangeal synos- 

 tosis. Neither in the Ai, nor the Unau, is there any thing to 

 interfere with free flexion and extension of the pes. 



The teeth are five in number on each side above, and four 

 below, and become sharpened by mutual attrition into a chisel- 

 like form. The stomach is remarkably complex. 



The Gravigrada are, for the most part, like the Sloths, 

 South American forms, but they are entirely extinct ; and 

 while, in most respects, they resemble the Sloths, in others 

 they present an approximation to Ant-eaters. 



The jugal arch may be complete or incomplete. The artic- 

 ular surfaces of the dorsal vertebras are sometimes complicated 

 in a manner similar to that observed in the Ant-eaters. The 

 tail is very long and strong. The limbs are short and sub- 

 equal, while the fore-foot has the ulnar digit imperfect, as in 

 the Ant-eaters. The fibula has no inward process, and the 

 astragalus is consequently devoid of any fossa upon its outer 

 surface. But another kind of peg-and-socket ankle-joint is 

 produced by the interlocking of the surfaces of the tibia and 

 of the astragalus. 



The great extinct animals. Megatherium^ Mylodon^ Mega^ 

 lonyx^ etc., the remains of which have been found almost wholly 

 ia later tertiary deposits of America, belong to this group. 



