440 



TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



compressed, and the zygoma is slender and decidedly 

 feeble, indicating that the skull was dolichocephalic. 



The total dimensions of the grinding series are, pre- 

 molars, 89 millimeters; molars, 152; premolar-molar 

 series, 242. This is of smaller size as compared 

 with the inferior grinding series of Protifanotherium 

 emarginatum (304 mm.), of P. superbum (318), and of 

 Telmatherium altidens (313). 



Progressive characters. — Among the details of pro- 

 gressive character may be mentioned : (1) The cingulum 



pas. Tiis. tr Aj 



Figure 369. — Type skull of Diplacodon elatus (female) 



Upper jaw and zygoma. One-fourth natural size. Yale Mus. 10320; Uinta C 1. 

 zygoma partly reconstructed; si, S2, sections; As, top view of zygoma, 



is continued on the inner sides of the crowns of the 

 premolar teeth; (2) the premolar teeth are rendered 

 quadrate internally by the expansion of the tetartocone 

 shelf, and p^-p"* exhibit the progressive development 

 of the tetartocone from before backward by constric- 

 tion or budding from the deuterocone crest; (3) the 

 fossette of the crown is seen in a rudimentary form in 

 the premolars and very distinctly in the molars, a 

 feature characteristic of all Oligocene titanotheres and 

 clearly foreshadowed in Rhadinorhinus diploconus of 

 the upper Eocene. 



Canines. — The canines are small, laterally com- 

 pressed, and slightly recurved, but too much worn 

 and fractured to clearly represent their form. 



Premolars. — As noted above, the deuterocone crest 

 is convex on the median or lingual side and flat or 

 concave on the outer or buccal side, as in Telmathe- 

 rium. Another distinctive character is seen in the 

 ectolophs, which are decidedly flattened, especially 

 the outer surfaces of the tritocones, reminding us of 

 the condition in Dolichorhinus; the external cingulum 

 arises and festoons the protocones. The detailed 

 proportions of the teeth (ap. by tr.) are, p^ 21 by 23 

 millimeters; p^ 25 by 29; j)\ 28 by 35. In p* the 

 deuterocone is large and elevated (17 mm.) and the 

 worn tetartocone is low (13 mm.) but almost entirely 

 distinct. 



Molars. — In the molars the external cingula are 

 faint, but the internal cingula are wanting. A 

 characteristic feature is the antero-internal expansion 

 of the cingulum, which makes the entire anterior 

 border angular and prominent, especially as seen in 



m^ This cingulum ridge bounds the fossa for the 

 metaconid and is clearly foreshadowed in Rhadino- 

 rhinus diploconus. The fossettes are nearly worn out 

 in m' and m^; both anterior and posterior fossettes 

 appear, and in m' the anterior fossette is a deep, 

 narrow pit. The proportions of the teeth (ap. by tr.) 

 are, mS 41 by 44 millimeters; m^, 55 by 55; m', 60 

 by 55. These proportions are decidedly different from 

 those in Protitanotherium. (Fig. 371.) The posterior 

 cingulum of m^ is elevated, and a low, distinct hypo- 

 cone swelling appears. The 

 ectolophs exhibit compressed 

 mesostyles. The ectolophs are 

 moderately hypsodont; the pos- 

 terior view of m^ shows that 

 the mesostyle terminates in a 

 horizontal ridge, as in many 

 little-worn or unworn titano- 

 there molars; it is especially 

 strong in Rhadinorhinus diplo- 

 conus. 



Comparison of Diplacodon ela- 

 tus with middle Eocene titano- 

 theres. — Diplacodon combines 

 characters of Telmatherium and 

 of Dolichorhinus but does not agree with either in all 

 respects. 



Comparison with Telmatherium: Diplacodon shares 

 with Telmatherium the tendencies toward dolichoce- 

 phaly and toward the development of tetartocones, 

 but it far outstrips Telmatherium in both features and 

 differs very radically in others, as follows: The canines 

 are small; p^ in Diplacodon is much more advanced 

 and of different type; the tritocones of the superior 

 premolars are flattened instead of rounded; the molars 



Ai, Upper jaw and teeth, with 

 partly reconstructed. 



Figure 370. — Third and fourth upper pre- 

 molars of Diplacodon elatus 



Natural size. Yale Mus. 10320 (type, reversed). Uinta 

 CI. Outer side view. 



are much more elongate and have flattened ectolophs; 

 the zygomata are much more slender. 



Comparison with Protitanotherium: These animals 

 are readily distinguished from the mesaticephalic 

 Protitanotherium in all the dolichocephalic propor- 

 tions of the teeth; and from the radical differences in 



