24 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I49 



the median plane of the skull. The area of each fossa is divided 

 about equally between the parietal and squamosal, although the 

 lambdoidal crests are for the most part formed of a fringe of the 

 exoccipitals. There is no evidence of an interparietal. In Phenacodus 

 the cranial portion appears relatively shorter than in Meniscotherium. 

 This is effected in part by the more gradual convergence of the 

 temporal crests resulting in a somewhat more posterior position of 

 the postorbital constriction. It may be further noted that the lamb- 

 doidal crests do not project out over the occiput so markedly and are 

 relatively broader in Phenacodus. Hyopsodus on the other hand 

 exhibits a rather elongate cranial portion, and the occipital crest 

 turns sharply upward, producing a more distinct dorsal concavity 

 in the posterior portion of each of the temporal fossae. An irregular 

 cluster of vascular foramina is noted near the parieto-squamosal 

 suture in the lateral portion of these depressions. 



Lateral view. — In lateral view (see pis. 1-3) the premaxilla of 

 Meniscotherium is seen as a narrow bar or plate extending from 

 the incisors upward and backward along the anterior margin of the 

 maxilla and wedging out posteriorly between the maxilla and nasal. 

 Only a short length of nasal extends free anterior to the premaxilla. 

 In uncrushed skulls the maxilla is moderately deep and exhibits a 

 large infraorbital foramen approximately above the contact between 

 the third and fourth premolars, well above the tooth row and well 

 forward of the orbital margin. The lachrymal bone extends for- 

 ward a short distance anterior to the orbital rim and upward al- 

 most but not quite to the nasals. In its juncture with the jugal 

 below it excludes the maxilla from the orbital rim. The lachrymal 

 foramen has a position essentially on the orbital rim to just within 

 the orbital fossa, where it may be partially concealed by the lachry- 

 mal tubercle on the rim. The anterior margin of the orbit is above 

 approximately the anterior margin of the second molar, not so far 

 forward as in Ectocion or the later hyracotheres, and the lower mar- 

 gin is not deflected outward on the jugal so noticeably as in those 

 forms. The jugal is moderately deep and forms a strong, dorsoven- 

 trally deep attachment to the maxilla, but the crest of the zygoma 

 does not carry so far forward on the face. Posteriorly the zygomatic 

 process of the squamosal is strikingly deep above the glenoid surface, 

 and the upper portion of the flare is turned decidedly inward toward 

 the crest of the ascending ramus of the mandible and the sagittal 

 crest of the skull. Posterior to the postglenoid process the crest of 

 the zygoma extends onto the cranium defining the lower margin of 



