554 



RODENTIA 



purtion of suture between squamosal and parietal in old 

 individuals, and occasionally a low, backward-projecting lambdal 

 ridge ; parietals abruptly bent downward at sides, giving the 

 upjier surface of brain-case a peculiar narrowed appearance ; 

 interparietal somewhat variable in form, but usually ligulate, with 

 pointed, rounded or squarish extremities and slightly projecting 

 median anterior angle, the antero-posterior diameter from one- 

 third to one-half transverse diameter ; auditory bulhe forming a 

 very conspicuous feature of brain-case when viewed from behind, 

 their area decidedly greater than that of occipital, the depth 

 through bulla about equal to distance between outer borders of 

 paroccipital processes ; foramen magnum rather large, broader 

 than high, its inferior lip about on level with tips of paroccipital 

 processes ; floor of brain-case without special features, the suture 

 between basioccipital and basisphenoid remaining open until late 

 in life ; auditory Iiullse laige, smoothly inflated, the area of each 

 IniUa somewhat greater than that of space enclosed between 

 zygoma and side of skull, the anterior border extending forward 

 to level of middle of glenoid fossa, the posterior border solidly 

 moulded against paroccipital jarocess and extending upward along 

 side of exoccipital considerably above level of zygoma, so that 

 this portion of bulla is plainly visible when skull is viewed from 

 above ; meatus large, not tubular, but with conspicuous, forward- 

 curyed lip in region nearest zygoma ; interorbital region slightly 

 hour-glass shaped, the upper surface flat or a little convex 

 laterall}-, the edges behind middle becoming somewhat angular in 

 old individuals ; zygomata rather weak, compressed, a little 

 expanded at middle, gradually spreading anteriorly, the greatest 

 breadth nearly at glenoid level, the lower border essentially 

 straight though sometimes a little concave ; anteorbital foramen 

 rather large, consideiably higher than wide, its greatest width 

 above middle ; rostrum almost parallel-sided when viewed from the 

 side, slightly tapering when viewed from above ; nasals a little 

 narrower posteriorly than anteriorly, the posterior border sti-aight 

 or emarginate, rarely pointed, usually about on level with 

 posterior termination of nasal branches of premaxillaries and 

 with middle of lachrymal ; incisive foramina large, narrow and 

 parallel-sided anteriorly, becoming abruptly much wider through 

 posterior half or two-thirds, the greatest width of both together 

 slightly more than half length, the length fully equal to one-half 

 diastema ; " a noticeable curved ridge extending from posterior 

 border of each incisive foramen to fi'ont of alveolus of corre- 

 sponding premolar ; palate wider anteriorly than posteriorly, 

 the small jDortion formed by the palatine bones conspicuously 

 fenestrate ; mesopterygoid space nearly parallel-sided, slightly 

 wider posteriorly than anteriorly, the least width about one-third 

 length, the anterior border rounded, with or without a small 



* Not Ions; enouoh in fig. 100. 



