640 



TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



scribed in its greater length (258 mm.) and heavier 

 shaft. 



The ulna of M. petersoni (Princeton Mus. 10013) is 

 slender (length 310 mm.) and slightly more curved 

 anteroposteriorly than in Palaeosyops. The olecranon 

 is relatively thicker transversely and is subtruncate 

 rather than pointed posterosuperiorly ; the dorsal 

 extension of the humeral facet is narrow. The antero- 

 external face of the shaft faces more obliquely out- 

 ward; this is partly because the external longitudinal 

 ridge is much lower than in Palaeosyops. The distal 

 end of the shaft curves backward more sharply. The 

 distal cuneiform facet is nearly at right angles to the 

 main axis of the shaft, and the external or styloid 

 process is not pronounced, whereas in Palaeosyops the 

 cuneiform facet is more inclined to the shaft, and the 

 styloid process is more pronounced. 



Manus (Am. Mus. 1571, Princeton Mus. 10013): 

 The chief characteristics of the manus in comparison 

 with those of Palaeosyops are its narrowness and 

 vertical height, the straight-sided character of the 

 metacarpals, and the shortness of the phalanges, so 

 that it represents the extreme dolichopodal stage 

 known among the Bridger titanotheres. It shares 

 several of these characters with Manteoceras, as well 

 as many of the more detailed characters of the carpals 

 and metacarpals, and differs from that form chiefly 

 in its greater narrowness. The general measure- 

 ments are given on page 636. In the measurements 

 given below more in detaU the first always refers to 

 Princeton Mus. 10013, the best-preserved specimen, 

 and the second to Am. Mus. 1571. The scaphoid is 

 relatively deeper vertically (25, 27 mm.), shallower 

 anteroposteriorly (36, 35), and broader posteriorly 

 (23) than in Palaeosyops (Am. Mus. 12205); the radial 

 facet is flatter, the trapezoid and scaphoid facets 

 form a more open angle (Princeton Mus. 10013 only); 

 the facet for the capitellum of the magnum is wider; 

 there is a close general agreement with the scaphoid 

 of Manteoceras (Am. Mus. 12204), save that the bone 

 is narrower and the trapezoid facet smaller. The 

 lunar is relatively narrow (27 mm.) and deep (32, 33 

 mm.) on both the anterior and posterior faces; the 

 inferior end is more sharply wedge shaped, the mag- 

 num facet being sub vertical in front view; the lower 

 facet for the cuneiform is broader. In Palaeosyops 

 the lunar is separated in front from the cuneiform 

 by the dorsal ridge of the unciform. The cunei- 

 form is narrow (26 mm.), not extended postero- 

 externally, with ulnar and unciform facets com- 

 paratively deep anteroposteriorly (Princeton Mus. 

 10013, crushed in Am. Mus. 1571); facet for pisiform 

 less elongate; cuneiform narrower than in Manteo- 

 ceras (Am. Mus. 12216). The pisiform contrasts in 

 many characters with that of Palaeosyops: the distal 

 end of its tuber is less expanded vertically and thicker 



transversely; its ulnar facet is concave and triangular, 

 that of Palaeosyops is convex and with rounded con- 

 tour; inferiorly its cuneiform facet is rounded, that of 

 Palaeosyops is deeply angulate; the head, or proximal 

 end, is sharply constricted from the shaft, that of 

 Palaeosyops rises gently from the shaft. The trape- 

 zium is small (greatest length 22 mm.), pear-shaped 

 (resembling a small patella) rather than broadly 

 ovate {Palaeosyops); a distinctive feature is that it 

 articulates with the scaphoid. Trapezoid relatively 

 wide (tr. 19 mm.), flat, anteroposteriorly shallow 

 (16 mm.), and more truncate posteriorly, not pro- 

 duced anteroposteriorly into an oblique projection; 

 also shallower anteroposteriorly than in Manteoceras. 

 Magnum (Princeton Mus. 10013) rather small, 

 scaphoid facet broad, flat, and subhorizontal; posterior 

 hook broadly spatulate (Am. Mus. 11659); facet for 

 Mtc II large and sharply ridged; magnum thus 

 agreeing in general with that of Manteoceras but 

 smaller and with narrower capitellum. The unci- 

 form is narrow (extreme width 37 mm.) and deep 

 vertically (diameter at right angles to long axis 

 25 mm.); lunar facet subquadrate, ridge separating 

 lunar from cuneiform low and not greatly produced 

 posteriorly; postero-external process with relatively 

 slender base and subpyramidal top, agreeing in facets 

 with unciform but entire bone narrower. As noted 

 above, the metacarpals (Am. Mus. 1571, Princeton 

 Mus. 10013) are elongate, straight-sided, subcylin- 

 drical rather then flattened, and more nearly parallel 

 with each other than in the spreading manus of 

 Palaeosyops; the distal facets are somewhat flatter 

 (less subglobose); the fifth metacarpal (Mtc V) is 

 relatively much longer and narrower. Distinctions 

 from Manteoceras are found chiefly in the greater 

 narrowness and in the obliquely triangular rather 

 than the posteriorly truncate broad proximal facet of 

 Mtc II and III. 



Considered more in detail: The first metacarpal, 

 as in other perissodactyls, is entirely wanting, imless 

 it is represented possibly by the distal part of the 

 trapezium. The second metacarpal (length 110 mm., 

 maximum distal width 25) has the trapezoid facet 

 deeply concave in front and produced postero- 

 iaternally into a blunt tip, imlike both Palaeosyops 

 and Manteoceras; the trapezium facet, as in Manteo- 

 ceras, is small and confined to the postero-external 

 border; the facet for the magnum forms an elongate 

 rectangular, nearly plane sin-face rather than an irreg- 

 ularly warped band, it is also more shallow posteriorly 

 than in Manteoceras; the facet for Mtc III is quite 

 small (contrast Palaeosyops). The third metacarpal 

 has the proximal facet (for the magnum) pointed 

 posteriorly instead of roundly truncate, as in Manteo- 

 ceras and Palaeosyops; the facet for Mtc II is very 

 small; the facet for the unciform is broadly triangular. 



