EVOLUTION OF THE SKELETON OF EOCENE AND OLIGOCENE TITANOTHERES 



625 



both trochanters somewhat elevated and nearly 

 opposite each other (an important point of distinction 

 from the rhinoceroses); shaft narrowing, with a 

 rounded anterior and flattened posterior surface 

 below trochanters; transverse measurements, distally 

 93 millimeters, across condyles 83; the internal and 

 external condyles are equally elevated but the internal 

 condyle is somewhat smaller; patellar facet moderately 

 elongate, vertically placed — that is, looking forward 

 and very slightly downward; vertical measurement 

 63 millimeters, transverse 40. This is the titanothere 

 type of femur, which, although varying in proportion 

 in the longer-limbed forms, is readily distinguishable 

 from that of Amynodon and of other contemporary 

 Perissodactyla. 



The tibia (Am. Mus. 

 1562) is distinguished by a 

 moderate development of 

 the cnemial crest; posteri- 

 orly the popliteal space is 

 bounded by a high internal 

 and a low external border; 

 the posterior surface of the 

 midshaft is gently convex, 

 the inner surface flattened; 

 anteriorly the cnemial crest 

 subsides into the angulate 

 anterior face of the shaft. 



The fibula is restored 

 from other specimens. It 

 has a very elevated postero- 

 external facet on the tibia, 

 and inferiorly it articulated 

 not only with the astrag- 

 alus but also in extreme ex- 

 tension of the pes it barely 

 touched the calcaneum. In 

 the contemporary aquatic 

 rhinoceros Amynodon the 

 tibia is relatively shorter, 

 the superior head of the 

 fibula is more inferior in 

 position, and distally the 

 fibula barely, if at all, touched the calcaneum. 



The pes is known principally from the associated 

 feet of Am. Mus. 1550 and metapodials and tarsus of 

 Am. Mus. 1582, all finely preserved. It is distin- 

 guished generically by the relatively short neck of the 

 astragalus, by the broad and relatively shallow cuboid, 

 navicular, and cuneiforms, and by the moderate 

 elongation of the metatarsals. 



The total height of the calcaneum is 107 millimeters 

 as compared with 63, the maximum width; the tuber 

 calcis is suboval in section, the long diameter being 



obliquely placed; in the extreme extension of the tibia 

 in some individuals this bone passes over posteriorly 

 onto the calcaneum; the fibula also has a calcaneal 

 facet. 



The astragalus exhibits characters of family value in 

 the arrangement of the ectal, sustentacular, and 

 inferior facets as shown in Figure 541. The cuboidal 

 facet has a broadly oval external border. The astrag- 

 alar trochlea measures 50 millimeters transversely, 

 while the depth of this bone on the internal side is 

 58; the trochlear groove is of moderate depth; on 

 the outer side the trochlear surface thins out poste- 

 riorly, becoming confluent with the actual astragalo- 

 calcaneal facet, allowing for the passage of the tibia 

 upon the calcaneum above mentioned; distally the 



inner side view of tarsus. Am. Mu 

 One-third natural size. 



Figure 540. — Left pes of Palaeosyops leidyi 



Am. Mus. 1589: Ai, Front view of pes with the phalanges foreshortened; A2, upper view of phalanges of the median digitfAa, 

 11682: Bi, Front view of pes; B2, upper view of phalanges of the median digit. 



neck of the astragalus measures 48 millimeters trans- 

 versely and 11 vertically. 



On its anterior face the cuboid measures 30 milli- 

 meters transversely and 23 vertically. The navicular 

 measures 44 transversely and 14 in its deepest part 

 vertically. The entire breadth of the second row of 

 the tarsi is 65, as compared with 55 in BTiadinorhinus 

 diploconus. The entocuneiform is well developed on 

 the postero-internal side of the tarsus, the mesocunei- 

 form is small, while the ectocuneiform is large, with 

 an internal facet for Mts II and an external facet for 



