THE OSTEOLOGY OF ELOTHEKIUM. 281 



in Sus. The superior border curves upward into a great, hook-shaped process, which 

 resembles that seen in Merycoehoerus, and gives a highly characteristic appearance to this 

 region of the skull. That portion of the zygomatic process which is directed anteriorly 

 is short and, though massive, is much less so than that which extends out laterally ; in 

 front it is received into a notch of the jugal. The glenoid cavity is large, transversely 

 directed and quite deeply concave, though the postglenoid process is not strongly devel- 

 oped and is hardly more conspicuous than the preglenoid ridge. This disposition is 

 unusual among the ungulates, but it occurs also in the Eocene genus Achcenodon and in 

 the modern Dicotyles. The glenoid cavities of the two sides are very widely separated, 

 their inner margins lying external to the line of the paroccipital processes. The posttym- 

 panic process of the squamosal is small, and is closely applied to the paroccipital process. 

 The shape of the zygomatic arches, together with the extreme narrowness of the cranium 

 proper, causes the temporal openings to be very large and to appear widely open when 

 the skull is viewed from above. These openings are, however, less extended transversely 

 and more antero-posteriorly than in Hippopotamus, while in Sus they are hardly visible 

 from above. 



The jugal is a very remarkable bone and constitutes one of the most extraordinary 

 features of the ElotJierium skull. Posteriorly it is notched to receive the zygoma, and 

 sends out a process along the ventral face of that bone, extending to the preglenoid ridge. 

 The jugal forms the inferior half of the nearly circular orbit, and for this purpose its 

 dorsal border is made deeply concave, giving off a stout postorbital process to meet that 

 of the frontal, while anteriorly it is moderately expanded upon the face in front of the 

 orbit, where it is wedged in between the lachrymal and the maxillary. The most pecu- 

 liar feature of the jugal, however, is the immensely developed vertical plate, which 

 descends from beneath the orbit downward and outward to below the level of the ven- 

 tral border of the mandible, recalling the similar, but much less massive processes found 

 in certain edentates, e. g., Megatherium. These plates are laterally compressed, but quite 

 thick, and when the skull is viewed from the front, they are seen to diverge quite 

 strongly downward ; their shape varies in the different species. In the very large forms 

 from the Protoceras beds, such as E. imperator, the process retains its plate-like form 

 throughout, its free end being only moderately thickened. This appears to be true also 

 of E. mortoni, though my material is not sufficient to allow me to make this statement 

 positively, but in the large species from the Titanotherium and Oreodon beds (E. ingens) 

 it forms a club-like thickening at the tip, which in E. ingens is coarsely crenulate on the 

 posterior border (see PI. XVII). These processes are, so far as is yet known, quite unique 

 among the hoofed mammals, and it is difficult to form even a conjecture as to what their 

 functional significance may have been. Some misunderstanding has arisen as to the spe- 



