132 



genus present also appreciable modifications of the astragalus : thus, in the 

 Mylodon Harlani the tuber at the anterior and internal side of the tibial articular 

 surface is longer than in the Mylodon robustus : this difference is indicated 

 indeed by the tibia. The generic modifications of the astragalus are of a more 

 striking character. In the Megatherium* it presents a more convex and deeper 

 external division of the upper articulation (a), and a relatively broader and shorter 

 internal tuberosity (6) ; but the more marked differences are observable in the 

 articulations for the naviculare and calcaneum. The upper half of the navicular 

 surface (c), which is flat in the Mylodon, is deeply excavated in the Megathere. 

 The calcaneal surface (e,/) is single and continuous with the navicular one in 

 the Mylodon, but is bisected by a deep rough canal in the Megathere ; the 

 posterior portion (/) being thus insulated, and the small anterior division (e) 

 alone being continuous with the convex part of the naviculo-cuboidal articu- 

 lation {d), as it is in the recent Sloths. 



An astragalus brought by Mr. Darwin from South America, and which may have 

 belonged to the Megalonyxf, agrees with that of the Megatherium in the depth of 

 the outer portion (a) of the tibial surface, and in the bisection of the calcaneal sur- 

 face, but resembles that of the Mylodon in the flatness of the upper half of the 

 navicular surface (c) . The astragalus of the Scelidotherium % agrees wuth that of 

 the Mylodon in the less depth of the outer division of the tibial surface (a) , and 

 the more open angle with which it joins the inner convexity (6) ; it agrees with 

 the astragali of the Megatherium and Megalonyx (?) in the division of the cal- 

 caneal surface (e, /) by a rough groove ; but it differs from the astragali of all 

 the other known Megatherioids by the presence of two deep concavities § upon 

 the naviculo-cuboidal surface, the portion to which the os cuboides articulates 

 being concave (c/) instead of convex, as in the other known Megatherioids. By 

 the astragalus alone, therefore, not only might the existence of a large Mega- 

 therioid animal be inferred, but also the particular genus to which it belonged 

 could be determined. 



Calcaneum. — As the modification of the calcaneal surface of the astragalus 



• Plate XXIII. figs. 5 and 6. f Ibid. figs. 3 and 4. ♦ Ibid. figs. 7 and 8. 



§ This character is well shown in the figure of the astragalus of the Scelidotherium leptocephalum 

 in the Fossil Mammalia of the Beagle, pi. 26, figs. 2 and 6 ; and in that of the Scelidotherium Cuvieri 

 in Dr. Lund's Blik, &c., pi. 4, fig. 3. 



