546 



RODENTIA 



of the depressions forming a distinct lambdoid crest, their inner 

 borders uniting to form a short sagittal ridge. Occipital region 

 almost squarely truncate, though sloping a little forward so that 

 the condyles are just visil:)le when skull is viewed from above 

 (unless hidden by unusual development of lambdoid crest), its 

 outline forming a broad arch about four-fifths as high as wide, 

 slightly flattened at the sides and above, the lower outer extremities 

 of the arch formed by the prominent, triangular, slightly in-curved 

 paroccipital processes, the j^oints of which descend nearly to level 

 of lower surface of condyles. Foramen magnum wider than 

 high. Floor of brain-case without special peculiarities ; a 

 slightly developed median longitudinal ridge : auditoi'y bulla 

 rather small, globular, the inflated portion longer than wide, 

 noticeably broader anteriorly than posteriori}-, the meatus with 

 a short though evident tube ; anterior border in contact with 

 hamular ; posterior border free from paroccipital process. 

 Interorbital region, the broadest portion of skull, highly arched 

 laterally, without evident ridges or depressions. Zygoma short 

 and heavy, not widely spreading, its anterior portion deep, the 

 posterior portion shallow, the lower border nowhere above 

 alveolar level thoiighsloping evidently ui^ward behind ; anteorbital 

 foramen considerably more than half as large as the rather con- 

 tracted orbit, its lower wall appearing as a conspicuous anterior 

 prolongation of zygoma. Nasals very large, their widest region 

 slightly behind level of posterior termination of nasal branches 

 of premaxillaries, their extension behind this region at least 

 equal to their anterior extension in front of anteorbital foramina, 

 the posterior outline of the two bones together evenly rounded, 

 their outer borders gradually converging anteriorly to a width a 

 little more than half greatest width ; anterior border of each 

 nasal strongly oblique with a slight but evident emargination at 

 outer edge. Nares somewhat naiTowly cordate in outline, the 

 apex projecting as a flat shelf or shallow trough over space 

 between shafts of incisors. Facial portion of maxillary 

 terminating posteriorly in line with posterior termination of 

 premaxillary and with middle of lachrymal. Palate narrow, 

 without special features, its width scarcely exceeding alveolar 

 width of cheek-teeth, the portion formed by palatine bones 

 relatively small ; incisive foramina small (length about 6 mm.), 

 narrow, slightly divergent posteriorly, nearer to incisors than to 

 cheek-teeth, their outer margins continuous with ridges which 

 pass backward to anterior borders of alveoli of premolars and 

 unite anteriorly to form a noticeable crest between foramina and 

 incisors ; septum dividing foramina continued backward as a 

 high median ridge flattening out rather abruptly at level of front 

 of premolar. Mesopterygoid space considerably more than half 

 as wide as long, narrowing rapidly forward from a little behind 

 m^', the evenly concave anterior border at level of front of 7ir' ; 

 hamular short, obliquely expanded, the posterior, longer limb of 



