618 



RODENTIA 



postorbital process forming a narrow but evident shelf, extending 

 from base of zygoma to beyond middle of posterior border of orbit ; 

 interparietal large, sub-quadrate, its antero-posterior diameter 

 about two-thirds transverse diameter ; lambdoid crest rather high 

 and distinct laterally, but scarcely noticeable along posterior 

 margin of interparietal ; occiput abruptly truncate, nearly 

 perpendicular below, slightly rounded-off above, the condyles 

 and paroccipital processes just visible in dorsal view ; seen from 

 behind the occiput is low and broad, its width noticeably more 

 than twice height above foramen magnum ; paroccipital processes 

 projecting more backward and outward than downward, their 

 lowest point slightly below level of condyles ; floor of brain-case 

 with no special features, the basioccipital with a moderately 

 defined median longitudinal ridge, its width along basal suture 



Fig. 122. 

 Letnmut, lemmus. Nat. size. 



contained about 2^^ times in median length ; auditory bulla large 

 but not very highly inflated, the interior nearly filled by a 

 dense mass of spongy tissue, the ventral border about on level 

 with cutting surface of molars, the size chiefly due to extension 

 forward into ectopterygoid fossa nearly to level of posterior 

 border of molar, width of bulla at level of meatus greater than 

 width between outer surfaces of occipital condyles. Inter- 

 orbital region narrow, cylindrical, scarcely as wide as rostrum, 

 its dorsal surface concave between zygomatic roots, posteriorly 

 with well marked ridges, which early unite to form a short, 

 trenchant median crest. Zygoma very heavy anteriorly, broadly 

 expanded at middle, abruptly slender and weak posteriorly ; ante- 

 orbital foramen rather small, very narrow below, the plate forming 

 its outer border slightly developed at base, its anterior border 



