EVOLUTION OF THE SKELETON OF EOCENE AND OLIGOCENE TITANOTHERES 



617 



oblique, magnum facet narrower, lunar facet narrower; 

 distal or phalangal facets of metacarpals and metatar- 

 sals more globose, less flattened, proximal facets of 

 same widely truncate posteriorly; tibia relatively 

 longer with narrower proximal end; astragalus with 

 narrower cuboid facet, navicular facet not so deep 

 anteroposteriorly, inner keel of astragalotibial trochlea 

 sharper, prominence on the distal part of the inner or 

 tibial surface not ending in a budlike tuberosity, 

 sustentacular facet less straight-sided. 



Forearm and manus from Bridger B 2, Grizzly Buttes west, Bridger Basin, Wye; 

 Am. Mus. 11699. Limnohyops ref. 



.Figure 527. (For measurements see above) 



This well-preserved specimen resembles in general 

 appearance the preceding one (Am. Mus. 11689) but 

 differs in the following particulars: The radius is a 

 little more slender; the olecranon of the ulna is deeper; 

 the manus is smaller but agrees in so many close 

 details with the other manus (Am. Mus. 11689) that 

 there can scarcely be any doubt that the two belong 

 to the same genus — namely, Limnohyops. 



The manus accordingly offers some further points 

 of contrast with that of Manteoceras — namely, in 

 the magnum the front face is not so sharply polyg- 

 onal, the posterosuperior head or eminence is much 

 narrower and extends obliquely backward and down- 

 ward, whereas that of Manteoceras is subtruncate pos- 

 teriorly and sharply ridged superiorly; the posterior 

 hook of the magnum is much more slender and ends 

 postero-inferiorly in a bluntly oval pitted surface; 

 the facet for the third digit, Mtc III, is deeper antero- 

 posteriorly and narrower posteriorly, that of Manteo- 

 ceras being subrectangular in general outline; the 

 unciform is narrower transversely and deeper verti- 

 cally, its supero-external or cuneiform facet is less 

 extensive transversely and narrower externally, it is 

 less concave in front view, its supero-internal or lunar 

 facet is less wide, especially posteriorly, all the ridges 

 between adjacent facets are less angulate; the postero- 

 external protuberance of the unciform is much more 

 pointed posteriorly; the cuneiform carpi are much 

 narrower transversely than in Manteoceras and reveal 

 many striking detailed differences. Some, or even 

 most, of the foregoing characters of the manus in 

 question may be primitive characters which have 

 been lost in Manteoceras in adaptation to the progres- 

 sive broadening of the manus; but if the manus in 

 question belonged to a direct ancestor of Manteo- 

 ceras manteoceras of the succeeding horizon we should 

 expect it to foreshadow that form a little more defi- 

 nitely, and the very well-marked differences indicate 

 again that it belongs to some other genus. 



Analogy to Mesatirhinus. — As compared with Mesa- 

 iirJiinus (fig. 526) this manus exhibits a rather strik- 

 ing general resemblance; but the carpals are broader, 

 the scaphomagnum articulation is more oblique, the 



metacarpals are broader, and the ungual phalanges 

 are larger and not so sharply flaring and truncate 

 distally. 



Conclusions. — This manus, although it is more 

 slender than that of Palaeosyops, shares with it certain 

 palaeosyopine characters in which it contrasts with 

 Mesatirhinus and Manteoceras, such as the greater 

 anteroposterior depth of the scaphoid, the angular 

 antero-internal border of the cuneiform, the pointed 

 rather than spatulate hook of the magnum, the 

 broader carpals, metacarpals, and phalanges. 



Hind limb from Bridger B 1, lower Cottonwood Creek, Bridger Basin, Wyo.; 

 Am. Mus. 11690. Limnoiiyops ref . 



Figures 516, 518, 519, 523, 686. (For measurements see above) 



The material includes only the rather well pre- 

 served left hind limb, namely, the femur, patella, 

 tibia, fibula, and pes. 



As compared with the hind limb of Am. Mus. 

 11689, described above, the present specimen differs 

 in its larger size and somewhat stouter proportions, 

 especially of the femur; the metatarsals are noticeably 

 longer and have straighter sides; in the astragalus the 

 sustentacular facet ex- 

 tends a little further dor- 

 sad, the ectal facet is less 

 deeply concave, the distal 

 calcaneal facet is larger, 

 and corresponding differ- 

 ences occur in the calcane- 

 um. These differences are 

 accompanied by so many 

 detailed resemblances 

 (in contrast with other 

 genera) that it appears 

 probable that the hind 

 limb in question belongs in 

 the same genus (Limnohy- 

 ops) with Am. Mus. 1 1689, 

 11699, above described, 

 though possibly to a differ- 

 ent species of that genus. 



As compared with other Palaeosyopinae the present 

 specimen offers many resemblances and a few rather 

 decisive differences: the femur, though smaller than 

 that of P. major, does not differ greatly in its propor- 

 tions and offers no clear-cut distinctions; the pes is 

 noticeably higher and more slender; the astragalus 

 is narrower with a narrower neck, more straight- 

 sided sustentacular facet, narrower cuboid facet 

 and sharper internal keel on the trochlea. 



Distinctions from Manteoceras-Dol ichorhinus 

 group. — From Mesatirhinus the specimen differs in its 

 larger size, more robust femur, tibia, and pes, distally 

 diverging vertical sides of Mts III. The astragalus 

 is wider, its ectal facet shallower and not produced 

 downward at the lower internal corner, the susten- 

 tacular facet was probably broader, not so straight- 



FiGURE 528. — Right scaphoid 

 of Palaeosyops sp. (A) and 

 Limnohyops monoconus (B) 



Front (Ai, Bi) and upper (As, B2) sur 

 faces. One-half natural size. 



