CASTOR 951 



long as transverse diameter of inflated portion of bulla, con- 

 spicuously ridged postero-externally, the main axis of flask 

 oblique but nearly upright, sloping outward at an angle of only 

 about 30° ; portion of bulla appearing on lower surface of skull 

 about 1J times as long as wide, evenly inflated, the width at 

 middle about equal to that of transverse diameter of basioccipital 

 pit measured inside the rim ; bulla separated posteriorly and 

 postero-externally from paroccipital and mastoid processes by 

 a broad irregular groove. Interorbital region short and wide, 

 imperfectly marked off from brain-case by the rudimentary post- 

 orbital processes. Zygomata heavy, gradually spreading, the 

 greatest zygomatic breadth about at glenoid level ; malar large, 

 greatly expanded at middle, its upper edge with projection 

 marking posterior limit of orbit, its depth through this projection 

 rather greater than diameter of orbit, its anterior extremity 

 firmly ankylosed with lachrymal ; anteorbital foramen minute, 

 slit-like or rounded, vertical, hidden in lateral view by the 

 oblique ridge which forms its outer margin and extends down- 

 ward toward alveolus of first cheek-tooth. Rostrum heavy and 

 deep, its least depth nearly equal to depth of brain-case at 

 middle, but less than distance from anteorbital foramen to 

 gnathion ; region between incisors and lower rim of nares con- 

 spicuously thickened ; nasals gradually narrowing from beforo 

 backward, their greatest combined breadth usually much less 

 than half length, their posterior border extending about to level 

 of middle of orbit, and decidedly behind ends of nasals branches 

 of premaxillaries ; incisive foramina slit-like, parallel sided, 

 situated somewhat nearer to incisor than to cheek-teeth, their 

 greatest length about one-third that of diastema. Palate narrow, 

 its width between anterior cheek-teeth about one-half that of 

 alveolus, its width between posterior cheek-teeth about twice 

 that of alveolus (the relative width of palate greater in immature 

 individuals) ; posterior border slightly behind level of w 3 , its 

 median spine well developed ; mesopterygoid space about two- 

 thirds as long as palate, its width posteriorly nearly three- 

 quarters its length, fully half its lateral border formed by the 

 long, robust, distally thickened hamular. Mandible very robust, 

 the ramus much thickened to accommodate the constantly grow- 

 ing roots of the cheek-teeth on outer side of large incisor-shaft, 

 the symphysis extending backward in adults to level of posterior 

 border of first cheek-tooth ; articular process very short, the 

 surface of the condyle scarcely extending beyond a line joining 

 tip of high, abruptly recurved coronoid process with hinder 

 extremity of evenly rounded-off angular process. 



Teeth. — Upper incisor heavy, its course scarcely indicated 

 on outer surface of rostrum, its root lying behind anteorbital 

 foramen ; shaft about as deep as wide, the anterior face slightly 

 curved, a little longer than sub-equal outer and inner faces, the 

 posterior border abruptly rounded ; enamel dark yellowish brown 

 in colour, essentially smooth but with very obscure longitudinal 



