1 82 THE SKULL. [CHAP. 



fossa. It thus forms the outer border of a large conical 

 cavity, opening posteriorly, bounded on the inner side by 

 the pterygoids, above by the alisphenoids, and below by the 

 palatines. 



The pterygoids are always simple subquadrate lamellae, 

 early ankylosed with the basisphenoids, often sending a 

 well-marked hamular process backwards, which unites with 

 the auditory bulla in Hystrix, Lagostomus, Bathyergus, &c. 

 There are usually well-marked pterygoid fossae, and the sides 

 of the alisphenoids are often perforated by an alisphenoid 

 canal. In Hystrix the hamular process is slightly bullate. 

 The squamosal forms a considerable part of the outside wall 

 of the cranium, but in consequence of the large size of the 

 united tympano-periotic, the root of the zygomatic process is 

 thrown very forward on the side of the skull, and the pos- 

 terior part of the body of the squamosal which unites with 

 the occipital is reduced to a long, rather narrow strip, 

 interposed between the parietal and periotic. When the 

 mastoid bullas are greatly developed, as in Pcdctes (see Fig. 

 59), this does not reach as far backwards as the occipital, 

 and is a slender curved process, which clasps the outside of 

 the bulla, and appears to hold it in its place. 



The glenoid fossa is situated on the under side of the 

 posterior root of the zygoma. In its most typical form (as 

 in the Capybara, Viscacha, Aguti, Paca, etc.) it is narrow and 

 concave transversely, with prominent inner and outer edges, 

 the latter being often formed, as before mentioned, partly by 

 the hinder end of the malar. In the Beaver the glenoid fossa 

 has considerable breadth. In the Porcupines, Marmots, 

 Squirrels, Rats, etc., no raised inner margin is developed, 

 and the fossa passes insensibly into the side of the skull wall. 

 In the Hare it is a transversely oval hollow, with a prominent 

 rounded anterior margin. 



