30 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 149 



not, however, have been fully ossified or possibly was loosely 

 attached and invariably lost. 



The glenoid surface for articulation with the lower jaw is broadly 

 concave transversely as well as fore and aft, with its longest diameter 

 extending anteroexternally. While this would appear to be nearly 

 a ball-and-socket arrangement, considerable forward motion of the 

 jaw is permitted. There is a prominent but relatively narrow post- 

 glenoid process, and posteromedial to it is a comparatively large 

 postglenoid foramen. It is interesting to note that in one specimen 

 (see pi. 1) the opening of the postglenoid foramen on the left is 

 through the squamosal but on the right its aperture is bounded 

 medially by a portion of the periotic. Posterior to the postglenoid 

 process the squamosal is broadly concave for the audital tube. 

 This space is bounded posteriorly by a rather weak mastoid process 

 which is nearly pinched out between the descending plate of the 

 squamosal and the very elongate paroccipital process. The striking 

 development of the paroccipital process is indicative of the signifi- 

 cance of the digastric muscle which would extend forward to the inner 

 part of the anterior portion of the lower margin of the jaw, possibly 

 also development of an occipito-mandibularis to the posterior margin 

 of the jaw as described by Sisson for the horse, which shows compara- 

 ble development of this process. 



The hypoglossal or condylar foramen is large and circular in out- 

 line and lies about in the middle of a depression between the par- 

 occipital process and the occipital condyle on each side. The fora- 

 men lacerum posterius is a more elongate, slitlike and medially 

 constricted aperture lying at the anteromedial root of the paroccipital 

 process and bounded forward by the petrosal. I suspect that the 

 constriction of this slit tends to define the jugular (more lateral) 

 portion as partially distinct from the somewhat wider part for nerves 

 IX, X, and XI. 



Although much of the Phcnacodus skull material is rather poorly 

 preserved or not completely prepared in the basicranial area, there 

 would appear to be a number of differences from Menisc other ium 

 worthy of comment. There is less evidence for a so well developed 

 pterygoid fossa in Phenacodus, suggesting less significance for the 

 internal pterygoid muscle. Its development in Menisc otherium would 

 probably correlate with the relatively deeper and more extended angle 

 of the lower jaw. The foramen ovale occurs just lateral to the pos- 

 terior extremity of the pterygoid flange but is relatively farther for- 

 ward, much closer to the alisphenoid canal and farther removed from 



