304 ON THE EXTINCT MAMMALIA OF 



dilFerent views of another, together with views of series of molar teeth, are given in 

 figures 14 — 21, plate x, and plate xi, of the Ancient Fauna of Nebraska. 



The specimens indicate a species about three-fifths the size of the Ancliiilierium 

 aurelianense of Europe. 



The skull of A. Bairdl bears a near resemblance to that of the Horse and Ass. 



The cranium is almost identical in form, proportions, and construction, with that of 

 the latter animals. The inion, including the occipital foramen and condyles, are the 

 same. The base of the cranium presents the same features in detail, so far as these 

 are preserved in the fossil and can be compared. 



The temporal fossae have the same form and constitution as in the Horse. They 

 are separated by a sagittal crest extending less than half their length. The squamo- 

 sals contribute a proportionately greater extent of surface in their construction. 



The face appears short in comparison with that of the Horse, mainly in consequence 

 of the proportionately larger size and advanced position of the orbits. . It is relatively 

 much shallower, in consequence of the less degree of development of the alveolar por- 

 tion in accordance with the comparatively short crowned molar teeth. The propor- 

 tionate breadth of the face is somewhat greater. 



The forehead has the same outline of form as in the Horse, but is much less 

 prominent. It is almost flat between the orbits, being feebly elevated laterally and 

 slightly depressed at the middle ; but farther back, between the acutely diverging 

 temporal ridges, it is somewhat convex. 



The frontal suture remains open in adult skulls, and the interparietal suture ex- 

 tends through the triangle whicli the parietals contribute to the forehead, or as far 

 back as the sagittal crest, as seen in figure 2. 



The orbital entrance is proportionately larger than in the Horse, and occupies a 

 more anterior position in the face. It has a nearly circular form, but is interrupted 

 posteriorly by a wide interval of communication with the temporal fossa. Instead of 

 the strong post-orbital arch of the Horse, A. Bairdi possesses merely a long, curved, 

 pyramidal post-orbital process to the frontal bone, as represented in figures 1, 2. 



The acute supra-orbital margin is situated but slightly below the general level of 

 the forehead. The anterior orbital margin is prominently everted and acute, and it 

 occupies a position on a line with the middle of the antepenultimate molar tooth. 



The infra-orbital arch is relatively about as well developed as in the Horse, but the 

 masseteric ridge extends but a comparatively short distance from it upon the maxil- 

 lary bone, as seen in figure 1. 



The face in advance of the orbits is proportionately smaller and more rapidly 

 tapering than in the Horse. 



A lachrymal depression, about like that of the Sheep, occupies the lower two-thirds 

 of the facial surface of the lachrymal bone, and extends forward upon the maxillary 



