.306 SCOTT. [Vol. V. 



The basi-sphenoid is shorter and more slender and tapering 

 than the basi-occipital, but otherwise like it in shape. The pre- 

 sphenoid is so covered by the expanded vomer that little of it can 

 be seen. The alisphenoid is of peculiar shape ; the ascending 

 portion is divided by an angulation into two portions, of which 

 the anterior is high, narrow, and nearly vertical ; and the pos- 

 terior forms a broad, flat, and nearly horizontal surface, rising 

 gently, to the glenoid cavity. The much more elevated position 

 of the latter in the horse gives to this part of the alisphenoid 

 a very different appearance. The orbito-sphenoid is high, but 

 rather narrow, not forming nearly so much of the wall of the 

 orbit as in the horse. 



The periotic has a high, but very narrow mastoid portion, 

 which is exposed between the squamosal and the exoccipital, and 

 forms a large part of the wall of the cerebellar fossa, as is plain 

 in a longitudinally bisected skull. The tympanic is not anky- 

 losed with the periotic, and in most specimens has become 

 detached and lost. It is very small and forms a minute bulla 

 with an elongated tubular meatus, and ending in front in a short, 

 sharp styliform process ; it encircles and apparently is co-ossified 

 with the tympanohyal. The periotic is in contact with the basi- 

 occipital, so that the large lacerated foramina are not contin- 

 uous, nor so extensive as in the horse. 



The squamosal is large, forming much of the side walls of the 

 cranium, but not rising so high relatively as in the horse ; the 

 zygomatic process is quite long and slender, and as in the horse, 

 extends forward so as to form part of the inferior rim of the 

 orbit. The peculiar and exceptional construction of the horse's 

 orbit is thus already indicated. The glenoid cavity is likewise 

 characteristically equine in structure ; its articular portion is very 

 narrow, slightly concave from side to side, and convex from 

 before backward ; its external portion is raised, and when viewed 

 from the side forms a prominent tubercle, which, however, is not 

 so broad, nor so conspicuous as in the horse. The post-glenoid 

 process is quite high, massive, and rugose ; between the post- 

 glenoid process and the articular surface there is a broad, shallow 

 depression. The post-tympanic process is short and inconspic- 

 uous, and is closely applied to the mastoid portion of the periotic. 

 The deep notch between the post-tympanic and post-glenoid 

 processes lodges the tubular auditory meatus. 



