547 



the expansion broadly in contact with inner portion of anterior 

 border of bulla. Mandible relatively slender, the posterior 

 portion not conspicuously deeper than ramus, including cheek- 

 tooth, the anterior portion a mere shell enclosing the robust 

 incisor ; articular process broad and low ; coronoid process small, 

 not rising conspicuously above level of posterior alveolar border, 

 widely separated from articular process ; angular portion broad, 

 lying conspicuously on outer surface of alveolus of incisor, its 

 posterior border slightly concave. Dental foramen at alveolar 

 level. 



Teeth. — Incisors robust, without special peculiarities of form, 

 the shafts scarcely compressed, but with antero-posterior diameter 

 perceptibly greater than lateral diameter ; enamel surface yellow. 

 Upper incisor with root extending to immediately in front of 

 alveolus of premolar, where its position is indicated by a slight 

 elevation on the surface of the bone ; 

 anterior face of tooth slightly oblique, 

 nearly flat though a little rounded off 

 at edges ; posterior surface narrow, 

 rounded, so that the outline of cross- 

 section is nearly an isosceles triangle 

 with all the angles rounded. Lower 

 incisor much longer and less curved 

 than upper, its root extending almost 

 to base of articular surface but not 

 producing any very noticeable swell- 

 ing on outer side of mandible, and 

 scarcely rising above level of molar 

 crowns ; anterior face of tooth very 

 oblique, scarcely marked off from pos- 

 terior face ; inner face flat. Cheek- 

 teeth flat crowned, rather large, not 

 showing any very noticeable contrasts 

 of size or structure among themselves. Roots very imperfectly 

 developed, smallest in the upper teeth where they appear as 

 mere irregular tubercles, three of which can usually be distin- 

 guished, and sometimes a fourth ; shafts higher than wide, 

 slightly tapering downward. Crowns of upper cheek-teeth 

 broadly elliptical in outline, distinctly longer than wide. Their 

 enamel pattern shows individual irregularities, but its scheme 

 is as follows : outer side of crown with three re-entrant folds, 

 the anterior and posterior of which soon become isolated as 

 narrow islands, the middle fold more persistent and often con- 

 fluent with re-entrant fold of inner side ; first and second folds 

 simple ; third curving backward and outward, its extremity 

 often becoming isolated as one or two minute secondary islands ; 

 lingual side of tooth with a single re-entrant fold usually directed 

 obliquely forward and often communicating with second or more 

 rarely with first outer fold. Pattern of premolar like that of 



2 n 2 



Fig. 108. 

 Hystrix cristata. Teeth. 



X 1|. 



