EVOLUTION OF THE SKELETON OF EOCENE AND OLIGOCENE TITANOTHERES 



597 



between EoTiippus venticolus and Hyrachyus agrarius, 

 as a transition between cursorial and mediportal 

 adaptation. The changing proportions of the astrag- 

 alus, calcaneum, and ectocuneiform are especially sig- 

 nificant. Attention should be called to Eotitanops as 

 isotridactyl — that is, the three metapodials are more 

 uniform in size than those of either Eohippus or 

 Hyrachyus. 



Peculiar features of the Eotitanops pes are (1) 

 elongate neck of astragalus; (2) a vertically elongate 



the distal ends of the metapodials. These gradations 

 correspond broadly with those we have observed in 

 the dental series and with the successive increases in 



Eotitanops major Osborn 



Type.^Am. Mus. 14894, a left median metatarsal 

 (fig. 493, D) ; also the distal end of the right tibia. 



Specific cTiaracters. — Of superior size; Mts III 104 

 millimeters longitudinal, 16 transverse, index 15. 



JSofiippus 



Jleptodo, 



Figure 502. — Left pes of cursorial and subcursorial Eocene Perissodactyla 



A, EoMppus venticolus, a primitive hippoid with narrow, slender foot and enlarged median metatarsal; B, Beptodon calciculus, a primi- 

 tive lophiodont with side toes not reduced; C, Eotitanops borealis, a primitive titanothere with broader tarsals and metatarsals 

 D, Hyrachyus agrarius, a primitive cursorial rhinoceros. One-half natural size. 



cuboid (inferential); (3) elevated ectocuneiform; (4) 

 Mts III with cuboid facet narrow or wanting; (5) 

 Mts IV with broad ectocuneiform facet; (6) meta- 

 podials keeled posteriorly; (7) proximal median 

 phalanges relatively short; (8) inferior astragalo- 

 calcaneal facets usually small, entirely separate from 

 sustentacular. The gradations in size of the as- 

 sociated or referred foot bones of Eotitanops are well 

 shown in the accompanying series (fig. 503) displaying 



This ill-defined species indicates the existence in 

 Wind Eiver time of a relatively large, slender-footed 

 titanothere, which is possibly ancestral to some of 

 the short-footed middle Eocene types. 



A comparison of the measurements of E. major with 

 those of E. horealis, from the Wind Eiver formation 

 (Am. Mus. 14888), indicates an animal of much larger 

 size but of the same foot proportions as shown by the 

 index. There is a small cuboidal facet. 



