NO. 2 A STUDY OF MENISCOTHERIUM — GAZIN 35 



The promontorium is not separately defined. The relatively large 

 fenestra rotunda faces backward, outward, and downward. Its pos- 

 terior rim is joined by a crest from the medial portion of the ventrally 

 exposed mastoid which also forms an anterior root of the paroccipital 

 process. This crest at the posterior rim of the fenestra rotunda may 

 also have supported a tympanohyal. Posteromedial to the fenestra 

 rotunda and anteromedial to the paroccipital process is a compara- 

 tively large foramen lacerum posterius. The relations here are closely 

 similar to those in Meniscotherium. Anterodorsal to the fenestra 

 rotunda is the smaller outward- facing fenestra ovalis. As in Menis- 

 cotherium the facial nerve would emerge from an aperture antero- 

 dorsal to the fenestra ovalis, descend to a position opposite the 

 fenestra ovalis, turn backward then downward to appear antero- 

 lateral to the root of the paroccipital process, were the otic area 

 covered by a bulla. 



The medial margin of the petrosal in Hyopsodus shows a broad 

 longitudinal groove partially formed by the lateral margin of the 

 basioccipital. The posterior extremity is determined by the point at 

 which the petrosal more solidly abuts the basioccipital, closer to the 

 foramen lacerum posterius than in Meniscotherium. This groove 

 clearly opens into the cranial cavity medial to the anterior extremity of 

 the petrosal and no doubt carried the internal carotid as interpreted for 

 Meniscotherium. 



The dorsomedial surface of the petrosal could not be seen in any 

 of the Hyopsodus material at hand without damage to the specimen. 



ENDOCRANIAL CAST 



An endocast of Meniscotherium was briefly mentioned by Edinger 

 (1956, p. 17) as being much smaller than that of Phenacodus but 

 of the same type. In addition to the specimen that she examined 

 (fig. 2, A.M. 48082), there are at hand two casts in the collections 

 of the U. S. National Museum. One of these (fig. 3, U.S.N.M. 

 23113) is a natural but somewhat eroded cast which includes the 

 rostral as well as the cranial portion. The other (fig. 4, U.S.N.M. 

 19509) was prepared in rubber from assembled cranial fragments, 

 but includes essentially only the dorsal surface, as its ventral surface 

 is incomplete posterior to the olfactory bulbs. Cranial fragments 

 of various other specimens present detail in restricted areas. 



The endocast of Meniscotherium, though actually smaller than in 

 Phenacodus, is relatively larger, as observed by Cope (1884b, p. 

 494). It appears elongate and slender, noticeably in the area of the 



