EDENTATA OF THE SANTA CRUZ BEDS. 2OI 



is invaded by a narrow V-shaped sulcus, which ends in a deep pit on the 

 neck for the reception of the tongue-like process of the tibia ; the fibular 

 facet is large and crescentic, as in Megalonyx, but with much more con- 

 cave plantar border, narrower proximal portion and much less prominent 

 everted process at the distal end. The external calcaneal facet is some- 

 what narrower than in Megalonyx, but much more concave and more 

 oblique to the proximo-distal axis, while the facet for the sustentaculum 

 is less distal and more internal in position and is separated from the 

 outer facet by a narrower sulcus. The neck is much shorter than in 

 Nothrotherium and even somewhat shorter than in Megalonyx, but has 

 a navicular facet like that of the latter, concave at the distal end and 

 reflected well over upon the tibial side. An important difference from 

 the last-named genus is in the position of the cuboid facet, which is a 

 convexity on the plantar side of the neck, while in the Pleistocene genus 

 it is on the fibular side. In the astragalus of Nothrotherium the most 

 striking difference from that of Hapalops is the much longer neck, which 

 is constricted by a deep notch on each side, ending in a convex head. 



The calcaneum also resembles that of Megalonyx, which Leidy de- 

 scribes as "having very much the appearance of the os ilium of some 

 smaller animal" ('55, 41). The tuber is long, but much less elongate 

 and with a far less slender neck than in the two Pleistocene genera, 

 expanding much more abruptly into a very broad, depressed, falcate 

 plate, which has a longer tibial than fibular border, while the proximal 

 border is much thickened and rugose, with prominent tubercles on the 

 plantar side ; the distal end is very heavy, with a great prominence on 

 the fibular side. A number of differences from the calcaneum of Nothro- 

 therium may be observed in the distal portion ; the external astragalar 

 facet is smaller, more simply convex and less reflected upon the dorsal 

 side; the sustentaculum is smaller and its facet is smaller and more 

 distal, less dorsal ; the cuboid surface is more oval. 



The navicular quite closely resembles that of Megalonyx, but is more 

 nearly circular and the low convexity on the proximal face, which fits into 

 the distal end of the astragalus, is less prominent; on the tibial side the 

 border is much raised, so that the proximo-distal length on this side is 

 more than twice as great as on the fibular; the large, convex facets for 

 the meso- and ectocuneiforms are more closely approximated than in the 

 Pleistocene genus, while that for the entocuneiform is entirely lateral. 



