G. T. BETTANY ON THE GENUS MERVCOCHCERUS. 267 



sharp projecting ridges which extend upwards to the transverse 

 occipital crest, enclosing between them a fossa of moderate size and 

 varying depth. Outside these ridges, on each side, is a much larger 

 and deeper fossa, bounded above by the lateral occipital crest pro- 

 ceeding obliquely downwards and forwards to the zygoma, below by 

 the occipital condyle of either side, and externally by the large 

 blunt convex postauditory process. Below this is the large par- 

 occipital process, which is triangular at the base, and extends down- 

 wards and somewhat inwards for an indeterminate distance closely 

 behind and externally to the auditory bulla. The smallest length 

 that can be allowed for the paroccipital process, consistently with the 

 size of its basal part and its length in Oreodon, is an inch and a half. 

 The external auditory meatus looks upwards and outwards in the 

 angle betwoen the posterior zygomatic root and the postauditory 

 process. 



The hinder parts of the zygomatic arch, the postglenoid processes, 

 the auditory bullae, and paroccipital processes are important features 

 in the widely extended posterior part of the undor surface of the 

 skull. The under surface of the basioccipital is convex, with a 

 strong median longitudinal ridge expanding into a blunt eminence 

 of considerable size, which is gradually lost anteriorly on the basi- 

 sphenoid. The latter bone, as well as the presphenoid, is smooth, 

 nearly flat, and narrow. The base of the cranium forms a very 

 regular curve, extending very much upwards and forwards. 



The auditory bullae, in the two specimens in which they have 

 been perfectly preserved, arise just in front of the foramina lacera 

 posteriora internally, and of the paroccipital processes externally. 

 Their hinder limit extends slightly behind a line joining the poste- 

 rior surfaces of the postglenoid processes. The bullae are large and 

 somewhat nipple-shaped and compressed, their long axis making an 

 angle of about 10° with the median line of the skull. The external 

 surface is more irregular than the internal, which may be perfectly 

 smooth. The large foramina lacera media are situated at the inner 

 anterior side of the base of the bullae, and are elongated parallel 

 with the bullae, so as to terminate acutely behind. Each foramen 

 is divided by a process projecting from the anterior boundary into a 

 smaller anterior and a larger posterior region. 



The glenoid articular surface, with its postglenoid process, is 

 situatod externally to the auditory bulla, separated from it and the 

 paroccipital process by a deep sigmoid fossa. The fore part of the 

 articular surface is nearly horizontally situated. It is slightly 

 convex antcro-posteriorly, shelving away in front and laterally, 

 where it is continuous with the under surface of the zygoma. A 

 posterior vertically descending portion is contributed to the glenoid 

 surface by the anterior flat smooth surface of the very large post- 

 glenoid process. This latter is transversely extended, somewhat 

 compressed antero-posteriorly, and strongly convex behind. It pro- 

 jects less deeply than the auditory bulla by a quarter or three 

 eighths of an inch. 



The under surface of the zygoma extends far outward beyond the 

 Q. J. G. S. No. 127. u 



