8 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 6l 



backward from the orbits do not coalesce on the parietals, but are 

 separated by a space of about 8 mm. A ridge which runs forward 

 from the occipital crest passes between these and finally disappears 

 on the general surface. However, in examining a number of skulls 

 of Grant's zebra, the cranium of which usually has a well-developed 

 crest, two are found with the post-orbital ridges separated to the 

 rear of the skull, but not so widely as in the Alaskan horse. One of 

 these two zebra skulls belonged to a young animal, the other to one 

 with the incisors well worn down. 



It is observed also that the hinder border of the palate extends 

 forward to nearly the middle of the second molars; while in the 

 type of Equus niobrarensis it reaches only to the hinder end of this 

 molar. 



The premaxillze in the Alaskan skull have the same absolute width 

 at the incisors that these bones have in the type skull. The outline 

 of the two jaws in this region are somewhat different, as seen from 

 above. 



The zygomatic arch, where narrowest, is 29 mm. wide in the 

 Alaskan skull; while in that from Nebraska this arch is absolutely 

 and relatively much wider, 37 mm. Its upper border above and 

 behind the glenoid fossa is much thicker and more obtuse than in the 

 type skull. The width of the occipital crest, at the suture between the 

 superoccipital and the parietals, is equal to 68 mm. in the Alaskan 

 skull ; in the Nebraskan, 74 mm. ; and the form is different in the 

 two ; especially does the occipital crest in the Nebraska skull project 

 more backward. 



The face of the type of Equus niobrarensis is more elevated than 

 that of the Alaskan skull, the midline being at a height of 123 mm. 

 above the rear of the palate, while in the Alaskan skull the height is 

 in mm. This is 5 mm. more than is required by the greater length 

 of the skull ; but slight distortion of the skull as restored, or individual 

 variation in either of the skulls, may easily account for the difference. 



There are some distinct differences found in the glenoid fossae of 

 the two skulls here compared. In the type of Equus niobrarensis the 

 articulatory surface has an extent, from side to side, of 63 mm. ; in 

 the Alaskan skull this is only 51 mm. In the Nebraskan skull this 

 surface is so concave from side to side that a straight line from its 

 extremities is, at one point, 8 mm. from the surface. In the case of 

 the Alaskan skull such a line is nowhere more than 4 mm. from the 

 surface. In the case of a considerable number of zebra and horse 

 skulls examined, this region shows much constancy in form. 



