NO. 2 A STUDY OF MENISCOTHERIUM — GAZIN 31 



the auditory chamber than in Menisc other mm. Moreover, the fora- 

 men ovale is not followed by a so well defined crest. The basi- 

 sphenoid and basioccipital are relatively broad and the ventral sur- 

 face appears somewhat more convex longitudinally. This may in part 

 be effected by the more prominent development of the area of muscle 

 attachment that would suggest better development of the rectus 

 capitis anterior major that functions in depressing the snout. The 

 glenoid surface for articulation of the lower jaw is not nearly so 

 concave and with less evidence of the fore and aft motion of the jaw 

 interpreted for Menisc otherium. The space between the postglenoid 

 process and the mastoid process, which would be occupied by the 

 audital tube, seems relatively shorter anteroposteriorly. The mastoid 

 exposure, however, between the weak mastoid process of the squa- 

 mosal and paroccipital process is much broader than in Menisco- 

 therium. The paroccipital process of Phenacodus, as noted, is much 

 less developed. The hypoglossal or condylar foramen is farther for- 

 ward in Phenacodus, as is also the auditory chamber with the petrosal. 

 The relatively elongate basicranium of Hyopsodus resembles that 

 of Meniscotherium in a general way, although there are noticeable 

 differences in detail. In the skull material available it would appear 

 that the pterygoid fossa was significantly developed, and as in Menis- 

 cotherium there is evidence that the lateral wall or descending ptery- 

 goid plate of the alisphenoid was stronger and better developed than 

 the pterygoid proper which defined the medial wall of the fossa. 

 The posterior margin of the pterygoid plate of the alisphenoid ascends 

 steeply to a point near or immediately in front of a somewhat elon- 

 gate aperture which may well be the confluence of the foramen ovale 

 and the posterior opening of an alisphenoid canal, if the latter is 

 present. This is decidedly different from that found in Meniscotherium 

 where the foramen ovale and alisphenoid canal are widely separated. 

 As in Meniscotherium there is a strong crest posterolateral to the 

 foramen ovale made up of plates from the alisphenoid and squamosal, 

 but more distinctly lateral in Hyopsodus and with less participation 

 of the alisphenoid. This leaves a much wider separation between 

 the crest and the foramen lacerum medium. A broad groove in the 

 alisphenoid for the eustachian foramen is relatively closer to the fora- 

 men lacerum medium and better separated from the crest or pedicle, 

 attributed to a possible bulla, than in Meniscotherium. There seems 

 rather less of the mastoid exposed posterointernal to this crest and 

 lateral to the tegmen tympani, suggested as a position of attachment 

 for the annulus in Meniscotherium. External to this crest and decid- 



