1910.] 



Characters of the Stem Perissodactyls. 



395 



globular nor flatly-covex but perhaps more or less intermediate in character. 

 The ungual phalanges were depressed, pointed and slightly fissured distally. 



From the preceding notes it appears that even in the stem Perissodactyls 

 the lunar (since its bluntly wedge-shaped distal end received the thrust of 

 both unciform and magnum, and its sides received the thrusts of the scaphoid 

 and cuneiform), formed the main keystone of the carpus (Fig. 25). The 

 interlocking carpus of any ungulate 

 consists, in fact, of a complex series 

 of keystones which receive the 

 weight and impact from the fore 

 arm above and the ground below. 



The astragalus in the ancestral 

 Perissodactyl differed from that of 

 Phenacodus in the following char- 

 acters : 



(1) The internal keel of the 

 trochlea was better developed. (2) 

 The neck of the astragalus was 

 more nearly parallel to the cal- 

 caneum and did not diverge distally 

 from it. (3) The head or condyle 

 of the astragalus was transversely 

 broader and flatter and its distal 

 facet (for the navicular) was warped 

 in two planes, /. e., convex antero- 

 posteriorly and very slightly con- 

 cave transversely, in contrast to the simple ovoid astragalar condyle of 

 Phenacodus. (4) The cuboid just touched the head of the astragalus 

 along a very narrow facet on its external angle. In Phenacodus the cuboid 

 is separated from the astragalus by the navicular, but this may be secondary, 

 because in Periptychids and some Creodonts the cuboid touches the astraga- 

 lus. (5) In the back view of the astragalus there was a small "distal facet " 

 (Osborn, 1889) by which the astragalus gained additional contact with the 

 calcaneum. This is absent in Phenacodus — apparently by reason of the 

 lateral divergence of the astragalar head from the calcaneum. (6) The 

 sustentacular facet, instead of being a broad median oval, was narrow, 

 and located more on the external border; it formed with the distal facet 

 a J . These characters of the astragalus may be seen in any Lower Eocene 

 Perissodactyl and sharply separate them from all known Condylarths. 



Fig. 25. Back view of the. carpus of an 

 Eocene Titanothere, Mesatirhinus petersoni 

 Osborn. The lunar rests almost equally on the 

 magnum and unciform. X f 



