84 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I49 



The glenoid surface for articulation with the lower jaw also has a 

 nearly human appearance in its sigmoid flexure, quite unlike Menis- 

 cotherium. It should be particularly noted that the jugal participates 

 in the glenoid articular surface and extends backward above this, 

 almost completely obscuring the zygomatic process of the squamosal 

 in lateral view. There would appear to be no tendency toward 

 reduction of the zygomatic process of the squamosal in Menisco- 

 therium. Anteriorly, however, the jugal is not so extended and does 

 not reach the lachrymal, as it does in Meniscotherium, so that the 

 maxillary forms a part of the orbital rim. 



With the more expanded brain case there is much less develop- 

 ment of the temporal or sagittal and lambdoidal crests, and the mas- 

 toid has a distinct lateral as well as occipital exposure. This bone, 

 however, is even more nearly excluded from the basicranial surface 

 than in Meniscotherium and is exposed only on the anterolateral 

 margin of the root of the paroccipital process, posterior to the 

 external auditory meatus. The hyracoid skull also retains a clearly 

 defined interparietal in fully mature specimens. This has never been 

 found in Meniscotherium or observed in any of the other condy- 

 larths for which skull material is known. 



The cheek teeth of the modern hyracoids are specialized somewhat 

 similarly to those in Meniscotherium, but the upper are more lopho- 

 dont and a little less crescentic. Moreover, the premolars have 

 become so completely molariform that few clues are left as to the 

 stages through which the pattern may have developed. Also, incisor 

 specialization has become highly distinctive. A more interesting 

 comparison may be made with the early Oligocene forms of Africa, 

 such as Megalohyrax and Saghatherium. While molarization of the 

 premolars is well advanced, as in the later forms, the upper molars 

 are rather more crescentic, particularly in the outer walls. A most 

 noticeable difference from Meniscotherium is the weak or obscure 

 character of the accessory cuspules in the upper series, particularly the 

 protoconule which is so well developed and crescentic in Menisco- 

 therium. In this respect the early hyracoids more nearly resemble 

 the Cernaysian Pleuraspidotherium, at least as far as the molars are 

 concerned. 



The number of presacral vertebrae in hyracoids is the greatest for 

 any of the living land mammals, and while Meniscotherium has one 

 more lumbar vertebra than Procavia, I suspect that the number of 

 dorsals was much less, as inferred from Phenacodus. The scapula 

 has a very different appearing spine, proximally subdued and entirely 



