GLIRES OF THE SANTA CRUZ BEDS. 



435 



widely separated from the mastoid processes. In the large species of 

 Perimys the periotic is exposed on the occipital surface, though less ex- 

 tensively than in Viscaccia, while in some, at least, of the small species, 

 it appears only in a narrow fossa between the exoccipital and squamosal ; 

 on the other hand, it is never visible on the cranial roof, as it so conspic- 

 uously is in Chinchilla and Lagidium. The periotic is also smaller than 

 in any of the three recent genera ; the mastoid process is quite well de- 

 veloped. The tympanic is larger than in Viscaccia and thus the auditory 

 bulla has quite a different appearance, and the meatus is a short tube, 

 which, compared with that of the latter genus, is directed more upward and 

 outward, less backward. 



The parietals are relatively longer than in Viscaccia and support a more 

 distinct sagittal crest, from which quite prominent temporal ridges diverge. 



FIG. 41. 



Skull of Viscaccia chilensis, x f . a. Left side. b. Top. 



The frontals are broad and have a vaulted, convex surface, due to the 

 large sinuses ; this inflated appearance of the forehead is more marked in 

 the larger species of the genus. The supraorbital ridge is prominent and 

 the postorbital process is much better developed than in Viscaccia and 

 the notch in front of it is correspondingly deeper ; in the modern genus a 

 small, separate bone is placed between the anterior ends of the frontals, 

 but I have not observed this in Perimys. The squamosal has a some- 

 what broader supramastoid process than in Viscaccia, but it is of quite a 

 similar shape ; between the anterior end of this process and the periotic is 

 a narrow, oval foramen, which is larger than in Viscaccia, but far smaller 

 than in Chinchilla or Lagidium. The jugal also is much as in Viscaccia, 

 extending behind to the glenoid cavity, and in front apparently rising to 



