EVOLUTION OF THE SKELETON OF EOCENE AND OLIGOCENE TITANOTHERES 



641 



whereas in Manteoceras it is intermediate; the facet 

 for Mtc IV is relatively smaller than ia Palaeosyops 

 and faces more downward than outward (Princeton 

 Mus. 10013); in Manteoceras it is intermediate. Mtc 

 IV has the proximal facet very different from that in 

 Palaeosyops; the unciform is flatter on top, and its 

 posterior part is not decurved so sharply; posteriorly 

 this facet is not so broad; the facet for Mtc III is 

 nearly divided into two triangular facets, whereas ia 

 Palaeosyops it forms a broad half ring; the facet for 

 Mtc V is much shallower; ia all these characters 

 Mtc IV approaches that of Manteoceras. The fifth 

 metacarpal offers a very wide contrast to that of 

 Palaeosyops; it is actually much longer (82 mm. as 

 compared with 75) while only about half as wide 

 (19 as compared with 36); the proximal end is pro- 

 duced externally into a high ridged prominence, which 

 embraces the unciform externally and causes the 

 unciform facet to face obliquely upward and inward; 

 the facet for Mtc IV is relatively narrow. In all 

 these characters except the extreme slenderness the 

 fifth metacarpal, lilve the fourth, approaches Manteo- 

 ceras manteoceras. 



Although the third metacarpal is considerably 

 loager than in Palaeosyops the first phalanx of the 

 same digit is only about two-thirds as long (24 mm.) 

 as that in Palaeosyops (37); it is, however, nearly as 

 broad (23, estimated) as it is long (24) and therefore 

 has about the same proportions as ia Palaeosyops 

 (ap. 37, tr. 36); this phalanx is thus proportionately 

 longer than in Manteoceras. The distal phalanx of 

 the same digit is widely spreading and sharply trun- 

 cate distally, with a fairly marked distal .cleft; in 

 these features it approaches the corresponding phalanx 

 in M. manteoceras but is longer in proportion to its 

 distal breadth (ap. 19 mm., tr. 28, as compared with 

 17 by 33 in No. 1587, M. manteoceras). The remain- 

 ing phalanges call for no special remark. 



Mesatirhinus petersoni? 



The manus Am. Mus. 11659, a part of the larger 

 skeleton from Bridger C 5, differs from those of 

 Am. Mus. 1571 and Princeton Mus. 10013 chiefly 

 in its larger size, as shown in the table of measure- 

 ments. It is pretty badly crushed but agrees well 

 in most details of the facets, etc. 



The hind limb is preserved only in the larger skele- 

 ton Am. Mus. 11659. The femur approaches that 

 of HyracTiyus, first, in the large size of the third tro- 

 chanter, which is relatively a little farther down the 

 shaft than ia Palaeosyops major; second, ia the position 

 of the patellar facet, which is more nearly at right 

 angles to the long axis of the bone, whereas ia Palaeo- 

 syops major it is prolonged upward and backward 

 and becomes nearly parallel to that axis. But these 

 characters are approached in the femur of Manteoceras 



aad besides being associated with a manus and an 

 astragalus of Mesatirhinus type, the femur itself is 

 separable from that of HyracTiyus by various differ- 

 ences in the shape of the head, great trochanter, and 

 distal end. The femur (length 358 mm.) is shorter 

 than in Palaeosyops leidyi (370 mm.), but its relative 

 length as compared ^with the tibia (T 79, F 100) is 

 the same as in Palaeosyops. The chief distinctions 

 from the femora of Palaeosyops, Telmatherium, and 

 Manteoceras lie in the greater slenderness of the shaft 

 and probably also in the position of the patellar 

 facets (see above). 



The tibia, though somewhat crushed, was long 

 (283 mm.) and slender; its distal third was relatively 

 steeper anteroposteriorly and narrower transversely 

 than in Palaeosyops major; a marked difference is 

 seen in the region of the astragalar facets; in front 

 view the facet for the internal keel of the trochlea is 

 deeply incised and bounded 

 internally by a promiaent 

 vertical malleolar process; in 

 iaferior view the same facet 

 is broad posteriorly, whereas in 

 Palaeosyops it is narrow pos- 

 teriorly; the facet for the ex- 

 ternal half of the trochlea is 

 deeper anteroposteriorly and 

 less produced antero - exter- 

 nally. 



The pes (Am. Mus. 11659), 

 like the manus, is of the long, 

 narrow type, with high tar- 

 sals and straight-sided meta- 

 tarsals. 



The astragalus (Am. Mus. 

 11659) is considerably larger 

 than the one that is associated 

 (Am. Mus. 1571) with teeth of M. petersoni type; the 

 navicular facet is also relatively deeper anteroposteri- 

 orly; the cuboid facet, the sustentacular facet, and the 

 neck all seem relatively a little wider. But notwith- 

 standing these differences, generic affinity is indicated 

 by the following characters in common, which serve to 

 separate these two astragali from those of other 

 genera. As compared with that of Palaeosyops 

 the whole bone is long (vertically high) and narrow, 

 with relatively narrower trochlea and neck; internal 

 or tibial keel of trochlea sharply rather than roundly 

 convex, inner slope of external trochlear keel flatter, 

 ridge bounding navicular facet superiorly not sharply 

 projecting, navicular facet shallower anteroposte- 

 riorly, cuboid facet narrower, less sharply inclined to 

 the long axis of the navicular facet; sustentacular 

 facet narrow, straight-sided, lying on the extreme 

 internal (tibial) side of the posterior face, broadly 



Figure 5 6 7. — Right 

 manus of Mesatirhi- 

 nus petersoni? 



Am. Mus. 11659; Bridger C 6. 

 One-third natural size. 



