642 



TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



continuous below with the cuboid facet, with which it 

 forms a well-defined L. The astragalus is distin- 

 guished from that of Manteoceras by the characters 

 noted above, chiefly the greater narrowness of the 

 bone as a whole, the sharper internal keel of the 

 trochlea, the narrower cuboid and sustentacular 

 facet. The astragalus approaches those of the 

 forerunners of LimnoJiyops from Bridger B (Am. 

 Mus. 11689, 11690), the principal differences noted 

 above being the relative narrowness of the bone as a 

 whole, the truncate inner border of the navicular 



Figure 568.^Left 

 femur and tibia 

 of Mesatirhinus 

 peiersoni? 

 Am. Mus. 11659; Bridger 

 C.5. Front view. One- 

 sixth natural size. 



Figure 569. — Left pes of Mesatirhinus 

 petersonif 



Am. Mus. 11659; Bridger C 5. Ai, Front view; 

 As, inner side view; A.i, phalanges of digit Ill- 

 front view; A*, ungual phalanx of digit III, 

 top view (a different individual from A3). 

 One-third natural size. 



facet, the somewhat narrower and straighter susten- 

 tacular facet. 



The calcaneum (Am. Mus. 11659) in its total length 

 (93 mm.) is nearly as long as that of P. leidyi (101 in 

 Am. Mus. 1589), but its total width across the susten- 

 taculum is only 47 millimeters as compared with 60 

 in P. leidyi; the tuber calcis as seen from above is 

 more straight-sided and its distal end less expanded 

 and rugose than in Palaeosyops; the sustentacular 

 facet is elongate-oval (vertical diameter 32 mm., tr. 

 13) rather than broadly oval, as in Palaeosyops. 



The navicular is nearly as deep vertically as in 

 Palaeosyops but is much smaller both anteropos- 

 teriorly and transversely; in superior view it forms a 

 quadrant. 



Aj (cH 



Figure 570. — Left astragali 

 of Mesatirhinus petersonif 



A, Am. Mus. 11659; B, Am. Mus. 1571. 

 Ai, Bi, Back view; A2, B2, front 

 view. Astragalocalcaneal facets; 

 ectal (ect), sustentacular (sus), and 

 inferior (inf). One-third natural size. 



The entocuneiform (Am. Mus. 11659) forms a vertical 

 oval (vertical 29 mm., tr. 22), which is broad below, 

 pointed at top, and obliquely truncate anterosupe- 

 riorly by the facet for the navicular; in Palaeosyops 

 (Am. Mus. 1589) this bone is much wider below, not 

 so high vertically, and ends 

 above either in a sharp 

 angle or in a low, rounded 

 hillock. The navicular 

 facet is subcircular, espe- 

 cially at its upper end, 

 whereas that of Palaeosyops 

 is either large and ovoid 

 (Am. Mus. 1589) or broadly 

 rounded (Am. Mus. 11682); 

 the facet for the meso- 

 cuneiform forms a decided 

 angle with the navicular 

 facet, while in Palaeosyops 

 it is more nearly in the same 

 plane. The lower end of 

 the posterior or internal 

 face shows two prominent 

 features — an oval facet for Mts II near the anterior 

 border, and near the posterior border a large, rounded 

 protuberance, probably for the attachment of a liga- 

 ment; this protuberance is wanting in Palaeosyops and 

 at first adds to the difficulty of homologizing the widely 

 different borders and facets in the entocuneiform of 

 the two genera. 



The mesocuneiform (Am. Mus. 11659) is a small, 

 anteroposteriorly elongate (20 mm.), narrow (10 mm.) 

 bone, in top view differing widely from the triangular 



mesocuneiform of Pal- 

 aeosyops. 



The ectocuneiform 

 (Am. Mus. 11659, 

 fig. 523) as compared 

 with that of Palaeo- 

 syops is deep vertically 

 (18 mm.; 19 in P. 

 leidyi), shallow antero- 

 posteriorly (33 mm.; 

 40 in P. leidyi), and 

 narrow transversely 

 (21 mm.; 25 in P. 

 leidyi) ; there are cor- 

 responding differences 



s. Am. Mus. 1550; B, M. peter- . t r J i 



soni.'. Am. Mus. 11669. Ai, Bi, Inner side ID- the laCCtS, and the 



view; A., B., outer side view. One-half antcro-external f a C C t 



natural size. ... 



for the cuboid is lacking. 

 The cuboid (Am. Mus. 11659) as compared with 

 that of Palaeosyops is also deep vertically (31 mm.) 

 and much more shallow anteroposteriorly (27 mm.); 

 the superior and inferior facets are subquadrate rather 

 than anteroposteriorly elongate. As compared with 

 that of Limnohyops from Bridger B (Am. Mus. 



A 2 



(TTzisH) 



Figure 57 L — Left entocuneiform 

 tarsi of Palaeosyops and 

 Mesatirhinus 



