EVOLUTION OF THE SKELETON OF EOCENE AND OLIGOCENE TITANOTHERES 



643 



11690) the cuboid is narrower transversely, with some- 

 what differently shaped facets and a larger postero- 

 external process. 



The metatarsals are long and straight-sided. The 

 measurements show that Mts III, although several 

 millimeters longer than in P. leidyi (Am. Mus. 1589), 

 is only -f^,; as broad near its distal end; the proximal 

 facet (for the ectocuneiform) is more truncate pos- 

 teriorly; as in the manus the distal facet of Mts III 

 is more transversely cylindrical and less convex 

 transversely than in Palaeosyops, but the distal facets 

 of Mts II and IV, being narrower, are more convex 

 or subglobose transversely. 



The proximal phalanx of Mts III as in the manus 

 is relatively longer (ap. 30 mm.), narrower (tr. 28 mm.), 

 and vertically shallower (17 mm.) than in Palaeosyops. 

 The middle or second phalanx, on the contrary, is 

 relatively wider (tr. 25 mm.) and much shallower (13 

 mm.). The distal or ungual phalanx of Mts III is 

 again rather long (27 mm.), narrow proximally (19 

 mm.), and very broad distally (28 mm.), the extreme 

 tip being transversely and the sides obliquely truncate. 

 This phalanx therefore differs widely from that of 

 Palaeosyops leidyi (Am. Mus. 1550), which is shorter 

 (23 mm.) anteroposteriorly and roundly spatulate 

 distally. 



Comparative measurements of the pes in DolicTiorJiininae, in millimeters 



Mesatirhimis 

 sp., Am. Mus. 



2352; 

 Washakie 



(B?) 



Mesatirhinus 

 petersoni?. 

 Am. Mus. 



11659; 

 Bridger C 5 



Metarhinus 



sp., Am.Mus. 



2058; 



Uinta B 2 



Dolichorhinus 

 hyognathus, 

 Am. Mus. 



13164; 

 Washakie B 



Dolichorhinus 

 hyognathus, 

 Am. Mus. 



1845; 

 Uinta B 2 



Astragalus, height of inner face 



Astragalus, breadth of trochlea 



Astragalus, height of sustentacular facet. 



Aftragalus, width of cuboid facet 



Calcaneum, height 



Calcaneum, width across sustentaculum.. 



Mts II, length 



Mts II, distal width 



Mts III, length 



Mts III, distal width 



Mts IV, length 



Mts IV, distal width 



65 

 50 

 39 

 11 



128 

 54 



53 

 40 

 35 

 9 

 94 

 48 

 110 



69 

 55 

 30 

 12 



147 

 37 



136 

 26 



120 



28 



110 



120 

 37 



108 

 21 



« Estimated. 



MesatirhJnus? 



A large unassociated pes from the Washakie Basin 

 (Am. Mus. 2352), of uncertain level, resembles in 

 many ways that of the referred MesatirMnus petersoni 

 (Am. Mus. 11659) described above but is much larger, 

 as shown by the measurements given below. The 

 proportions tend to dolichopody. As compared with 

 that specimen the astragalus is a little broader in 

 proportion to its height, and the cuboid facet is a 

 little more prominent, both progressive characters; 

 the sustentacular facet agrees with that of Mesa- 

 tirhinus and contrasts with those of Dolichorhinus and 

 Metarhinus in its long, straight-sided character; but 

 it is sharply separated from the cuboid facet, a very 

 exceptional condition. The calcaneum has the stout 

 neck, unexpanded head, and narrow sustentaculum 

 characteristic of Mesatirhinus. The third and fourth 

 metatarsals and the second phalanges of D. Ill and 

 D. IV agree closely with those of Mesatirhinus save 

 in the larger size. The pes is thus distinguished from 

 those of Dolichorhinus hyognathus and Metarhinus by 

 the greater length of the metatarsals and by the char- 

 acter of the sustentacular facet of the astragalus; it 

 is also distinguished from that of Manteoceras by the 



characters of the astragalus, in which the internal 

 keel of the trochlea is narrowly rather than broadly 

 convex, the cuboid facet is relatively narrower, the 

 sustentacular facet vertically longer, and the neck not 

 so short. 



The pes, therefore, appears to represent a large 

 species of Mesatirhinus perhaps allied to Dolichorhinus 

 vallidens. 



The question of the genei'ic reference of this pes is one 

 of great morphologic interest, which must be finally 

 determined by the discovery of another skeleton. 

 Its detailed characters (see fig. 572) are as follows: 

 (1) General proportions high and narrow; (2) Mts II 

 measures 145 millimeters, as against 118 in Dolichorhi- 

 nus hyognathus; (3) Mts IV measures 136 millimeters 

 ascompared with lOSinD. hyognathus; (4) the astrag- 

 alus exhibits an exceptionally high and narrow sus- 

 tentacular facet separated inferiorly from the small 

 inferior facet; (5) the calcaneum exhibits an elongated 

 tuber calcis (138 mm. as compared with 114 in D. 

 hyognathus). 



Another large pes from Washakie B (Am. Mus. 

 13175) is of the same dolichopodal type and appar- 

 ently of the same species. It is certainly a member 



