604 



TITANOTHEEES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



relatively broad type (mediportal and graviportal), 

 never elongate (cursorial) as in the primitive horses; 

 (5) the median phalanges are short; (6) the distal 

 phalanges expand at the extremities and exhibit deep 

 median clefts, indicating the attachment of broad 

 horny hoofs like those of the rhinoceroses rather than 

 narrow or appressed hoofs of the equine or hyracodont 

 type. The phalanges differ considerably in length 

 and in breadth in different genera. 



Manus oj the palaeosyopine group. — The palaeosyo- 

 pine group includes three genera, TelmatJierium, 

 Limnohyops, Palaeosyops; the manus of the last two 

 named is known. Of these Limnohyops is more 

 primitive and mediportal in its proportions and articu- 

 lations and is thus analogous to the primitive and 

 mediportal Mesatirhinus of the Dolichorhininae. 



-Characteristic details of radius and 

 upper Eocene titanotheres 



ulna in middle and 



Lower row, proximal end of left ulna, outer view. Middle row, distal end of left radius, front 

 view. Upper row, proximal end of left radius, front view. A, Limnohyops monoconus?; 



Limnohyops laticeps; C, Palaeosyops cf. 

 tirhinus petersoni: G, Dolichorhinus hyognathus. 



leidyi; D, Palaeosyops Tobustus; 

 One-sixth natural size. 



Figure 512 shows how similar these two forms are, yet 

 a very close examination of the details of structure in 

 the carpals and tarsals proves beyond question that 

 they belong in separate subfamilies and diverged from 

 each other at a remote period. 



The following dry descriptive details are of philo- 

 sophic interest to the comparative anatomist because 

 they demonstrate the assertion made above that we 

 can discover a distinct syngenetic (common origin) 

 character in each element of the carpus when closely 

 examined and compared. They prove that the rela- 

 tively light-limbed and more rapidly moving Limno- 

 hyops retains more of the ancestral form and propor- 

 tion (thus analogous to Mesatirhinus) while the short- 

 footed Palaeosyops diverges most widely in the gravi- 

 portal direction (thus analogous to Manteoceras) . 



Limnohyops characters. — Of mediportal proportions; 

 mesatipodal; carpus more elevated and compressed 

 than in the subgraviportal Palaeosyops; metacarpals 

 and digits relatively longer than in Palaeosyops but 

 relatively broader than in Mesatirhinus; digit V rather 

 short; scaphoid and cuneiform relatively high; lunar 

 high, resting mainly on unciform, obliquely on mag- 

 num; magnum high, laterally compressed; second 

 phalanges abbreviated; terminal phalanges cleft and 

 spreading distally as in Mesatirhinus. 



There are two subtypes of Palaeosyops manus. 

 (See figs. 537, 549.) 



Palaeosyops characters (Am. Mus. 12205). — Of 

 graviportal proportions; brachypodal; carpus broad 

 but less depressed than in Palaeosyops frohustus; 

 digits more abbreviated and spreading than in Palaeo- 

 syops leidyi; trapezium large, with no scaph- 

 oid articulation apparent; magnum relatively 

 high, rather broad, with but five distinct 

 faceted angles; the lunar facet in front view 

 is continuous with the unciform; terminal 

 phalanges irregular, rounded distally. 



Manus oj the Manteoceras-Dolichorhinus 

 group. — In this subfamily Mesatirhinus is the 

 primitive and mediportal type analogous to 

 Tapirus and Limnohyops, while Manteoceras 

 is the modified subgraviportal type analogous 

 to Palaeosyops, although less extreme. Close 

 comparison of the carpals of Mesatirhinus 

 and Dolichorhinus demonstrates the manteo- 

 ceratine affinity and divergence from the 

 palaeosyopine type. 



Mesatirhinus. — A mediportal carpus or 

 relatively high, narrow fore foot, all the 

 elements being vertically elongated but less 

 so than in Tapirus terrestris. Mesatipodal; 

 total breadth of carpus of a typical specimen 

 {M. megarhinus) 65 millimeters, total depth 

 43 ; scaphoid less deep anteroposteriorly than 

 in Palaeosyops, appearing relatively high and 

 narrow; lunar very high, with oblique magnum facet, 

 thus in front view bearing principally on unciform; 

 trapezium narrow, with scaphoid, trapezoid, and Mtc 

 II facets; trapezoid relatively large; magnum high, 

 relatively narrow, and subquadrate, with high facets, 

 hook of magnum more symmetrical, broadly spatulate, 

 with deep median groove; unciform with its longest 

 diameter oblique instead of horizontal, as in Palaeo- 

 syops; Mtc V elongate, manus consequently more 

 tetradactyl; metacarpals decidedly slender but less so 

 than in Tapirus terrestris; distal phalanges elongate, 

 cleft, and broadly expanded distally. 



Dolichorhinus. — The manus of Dolichorhinus repre- 

 sents a sub-brachypodal specialization of the Mesati- 

 rhinus type, paralleling that of Manteoceras. (See 

 p. 606.) 



