610 



TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



thus presents much more anteriorly than in the early 

 rhinoceroses {Amynodon). This indicates that the 

 femur was carried vertically at an early period. In the 

 manteoceratines the patellar trochlea is more distal 

 and oblique; thus in the more light-limbed Mesati- 

 rhinus the patellar facet presents more obliquely 

 downward and forward, and the femur was carried 

 more obliquely, as in the rhinoceroses {Hyrachyus 

 and Amynodon). Even in Manteoceras it was more 



downward than in Palaeosyops. Distally the internal 

 and external tibial condyles are nearly subequal. In 

 the amynodonts the external condyle is much more 

 prominent, and the corresponding external tibial facet 

 is produced backward. 



Femoi'otibial ratios. — In all the titanotheres the 

 tibia is much shorter than the femur. This disparity 

 is progressive (tibiofemoral ratio, 77 to 54) in the 

 course of graviportal adaptation. 



Length of hones of Mnd limbs oj titanotheres and of tapir, in millimeters 



Tapirus indicus 



Eotitanops princeps, Am. Mus. 296 (t3'pe)__ 



Palaeosyops major, Am. Mus. 1316 



Palaeosj'ops leidyi, Am. Mus. 1544 (type).. 



Limnohyops sp., Am. Mus. 11689 



Manteoceras manteoceras, Am. Mus. 1587.- 

 Mesatirhinus "petersoni," Am. Mus. 11659. 

 Dolichorhinus hyognathus, Am. Mus. 13164 



Menodus trigonoceras, Munich Mus 



Brontops robustus, Yale Mus. 12048 (type). 

 Brontotherium gigas, Am. Mus. 519 



320 



250 



433 



370 



355 



390? 



358 



386 



770 



812 



780 



258 



332 

 290 



285 

 272 

 285 



120 

 86? 

 137 

 110 

 111 



37 



34? 



31 



30 



31 



430 



448 

 427 



118 

 119 

 220 

 212 

 200 



33 

 30 

 28 

 26 

 20 



Contrasts with Amynodontidae. — The amynodonts 

 are contemporary semiaquatic rhinoceroses. In Amyn- 

 odon the great trochanter is more elevated ; the patellar 

 facet is subhorizontal at the distal extremity of the 

 shaft. 



TIBIA 



The tibia (fig. 516) is invariably shorter than the 

 femur, the ratios in the various genera being as in the 



FiGTiRK 517. — Distal end of the femur in a middle 

 Eocene titanothere and an upper Eocene amyno- 

 dont 



A, Manteoceras manteoceras; B, Amynodon sp. One-third nat- 

 ural size. 



above table. So far as the relative abbreviation of 

 the tibia is indicative of speed and weight, Manteoceras 

 is relatively the slowest and Mesatirhinus relatively 

 the swiftest of the middle Eocene titanotheres. 



Among the special titanothere characters are the 

 following: (1) External tuberosity of cnemial crest 

 more prominent than internal tuberosity; (2) cnemial 

 crest concave superiorly and deeply excavated on the 

 external side; (3) shaft deeply trihedral in midsection 



and broadly flattened inferiorly. Characters 2 and 3 

 persist in the Oligocene titanotheres. 



As compared with the large contemporary Rhinoc- 

 erotoidea (amynodonts and hyracodonts) of middle 

 Eocene time, some of which attained the same size 

 as titanotheres, we observe the following distinctions: 

 (] ) In Palaeosyops the cnemial crest runs more 

 obliquely inward, crossing the shaft; (2) the crest is 

 broader at the summit and does not extend so far 

 down the shaft as in Hyrachyus and Hyracodon; (3) the 

 tibia of Palaeosyops and Manteoceras in proximal and 

 distal views is shallow anteroposteriorly. 



As compared with the tibia of Amynodon, the titano- 

 there tibia is more robust and lacks the posterior 

 prolongation of the femoral condyle. 



The fibula was slender (but relatively less so than 

 in Amynodon), subcylindrical, broadly expanded above 

 for the articulation of the posterior inferior surfaces of 

 the external head of the tibia, and closely appressed 

 with the tibia below by a broad articulation, articu- 

 lating broadly also with the astragalus and exhibiting 

 a postero-inferior facet for the calcaneum during the 

 extreme extension of the foot. 



In the pes, as in the manus, we observe certain 

 syngenetic family characters which are peculiar to all 

 titanotheres but which in various genera are more or 

 less concealed by adaptations to speed or to weight. 



