34 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I49 



The dorsomedial surface of the periotic shows a relatively large 

 internal auditory meatus, directly opposite the promontorium on the 

 ventrolateral surface. Above the horizontal superior rim of this open- 

 ing is a large, obtuse conical depression for the flocculus of the cere- 

 bellum. At the apex of this conical surface in its posteroventral 

 portion there would appear to be a foramen of variable size. 

 Possibly this is the aqueductus vestibuli, but this may be located in 

 a more customary position directly posterior on the rim or crest of the 

 floccular fossa, where there also seems evidence for a foramen. On 

 the broad posteroventral margin of the petrosal, posteroventral to 

 the internal auditory meatus is a very large aperture which inward 

 is reduced to a rather small foramen that may well be the aqueductus 

 cochleae. The hiatus Fallopii for the superficial petrosal nerve is 

 evidently at the anterior apex of the petrosal. 



The petrosal seems relatively small in Phenacodus, the actual 

 size in large P. primaevus being only a little greater than in M. 

 robustum. It is clearly a little thicker and broader through its mid- 

 section, but its length to the anterior apex is relatively less. The 

 general plan of the ventrolateral surface is rather alike in the two, 

 with the position of the fenestrae and the details of the facial canal 

 and sulcus being not greatly different. The aperture of the facial 

 canal, however, is situated a little deeper anterodorsal to the fenestra 

 ovalis ; also the hiatus Fallopii, rather large in one of the two speci- 

 mens examined, is located more medial to the apex, rather than at 

 the apex as observed in M. chamense skulls. In the posterior portion 

 the fenestra rotunda is more widely separated from the foramen 

 lacerum posterius, and the facial sulcus swings around more pos- 

 terior to this fenestra as it turns more gradually downward in Phena- 

 codus. 



The inner or dorsal surface of the petrosal in Phenacodus shows 

 no striking differences, with the internal auditory meatus large and 

 similarly placed. The aqueductus cochleae shows the same wide 

 aperture to a small foramen but is possibly a little more dorsal along 

 the posterior margin. The aqueductus vestibuli seems decidedly slit- 

 like at the posterior margin of the dorsomedial surface, entering the 

 posterior part of the crest which defines the lower margin of the 

 floccular fossa. In the material at hand I was unable to determine 

 the presence or absence of a foramen in the depth of the floccular 

 fossa noted in Meniscotherium. 



The petrosal in Hyopsodus appears long and slender in ventral 

 view, as a slender cone with its apex directed forward and medially. 



