34^ SMITHSONIAN MISCELLANEOUS COLLECTIONS [vOL. 45 



are, the absence of postorbital processes, the general flatness of the 

 brain-case, the much greater development of the posterior free ex- 

 tremity of the malar, the shortness and slenderness of the rostrum, 

 the greater interorbital construction, and the differences in the 

 shape of the palate, all of which are best seen in the figures. The 

 fenestration of the maxillary bone in the Leporidse is replaced in 

 the Ochotonida: by a single large opening in the side of the bone. 



The mandible of the Ochotonid^e, although presenting many 

 differences from that of the Leporidse, is built on the same general 

 plan. In Ochotona the ascending ramus is relatively much wider. 

 The groove on the anterior surface of tlie ramus and the thin plate 

 of bone forming the outer border of this groove in the hares is not 

 found in the pikas. Just below the middle of the anterior surface 

 of the ascending ramus of the mandible of OcJiotoiia is a more or 

 less prominent tubercle that is lacking in Lcpus and its allies. 



Corresponding to the shorter rostrum of the pikas, that portion 

 of the mandible between the series of grinding teeth and the incisors 

 is relatively shorter than it is in the hares. 



The mental foramen in the Leporidse is situated on the side of the 

 mandible just anterior to the insertion of the first cheek tooth ; in 

 the Ochotonidse the mental foramen is located on the side of the 

 mandible situated as far posteriorly as the last lower molar. The 

 few skulls of the genus Ochotona that are available for study show 

 striking differences among themselves and may be placed in three 

 groups, which will be described further on as subgenera. 



The brain-case is generally flat and not rounded in Ochotona^ 

 especiallv so in the North American species, where the whole skull 

 is also flat. In the group represented by OcJiotona ladaccnsis the 

 skull as a whole is not flattened, although the brain-case is ; the 

 interorbital region is much constricted and highly arched. In the 

 group containing Ochotona roylii, the brain-case is less flattened 

 than in the other and more typical forms ; it is somewhat rounded, 

 suggesting the brain-case of the Leporidse. 



Two general styles of incisive foramina are found in the Ochoto- 

 nidge : (a) incisive foramina resembling those of the Leporidse in 

 general shape, but at the same time encroaching much farther 

 back on the bony palate, represented by Ochotona roylii; (b) in- 

 cisive foramina constricted into two unequal portions by the ap- 

 proximation, if not actual union, in the median line, of the posterior 

 ventral portions of the premaxillse, the anterior portions of the 

 foramina being very small, and the posterior portions very large- 



