Peterson: The Osteology of Promerycochcerus. 159 



gently and regularly convex fore-and-aft and is bounded posteriorly 

 by the heav}'- and truncated jiostglenoid process. The latter is 

 farther separated from the paroccipital process in this species than 

 in Promerycocha'rus chelydra (Compare Plate XXXVIII, Fig. 2 and 

 Plate XLI, Fig. 2). 



No suture is discernible between the squamosal and the temporal 

 bone. The latter takes up only a small area of the basi-cranial surface. 

 The typanic bulla does not extend as low as the postglenoid process, 

 and its antero-posterior diameter is 30 mm., while the transverse 

 diameter is 20 mm. ; its shape is thus sub-ovate with a dented and 

 uneven external surface. In P. chelydra the bulla is more conical and 

 extends downward even with the post-glenoid process. The paroc- 

 cipital process extends well below the tympanic bulla and is appressed 

 closely against the latter on its postero-lateral border. The external 

 auditory meatus is moderately large and is directed upward and 

 outward. The region between the bulla and the exit of the ear is 

 well protected by the mastoid portion and the base of the paroccipital 

 process. 



The lateral diameter of the area occupied by the basisphenoid is 

 considerable, and the under surface of this bone is without a median 

 keel. The foramen ovale is located close to the anterior edge of the 

 tympanic bulla and is of moderately large size. The foramen rotun- 

 dum is well back and well hidden from view by the external pterygoid 

 process of the alisphenoid. The external pterygoid fossa is narrow and 

 deep and apparently continues uninterruptedly forward joining the 

 orbital fossa with a very faint separating ridge of bone on the sphenoids. 

 The median pterygoid fossa is wide, the internal surface of the ptery- 

 goid process is concave at the base, which adds considerably to the 

 width of the fossa posteriorly. The inferior border of the pterygoid 

 process is rounded and rather heavy, descending at a slight angle to 

 meet the posterior part of the palatine. The posterior narial opening 

 is narrow anteriorly, which is partly due, in the specimen (No. 109), to 

 a slight lateral crushing in the posterior part of the palatine plate. 

 In this specimen the anterior border of this opening is opposite the 

 extreme posterior lobe of m^. This character is apparently variable, 

 as some individuals of the species have the anterior emargination of 

 the posterior narial opening further back than in the specimen under 

 description. I have not been able to locate the optic foramen. 



The posterior portion of the frontal region is sharply elevated to 



