182 PALAEONTOLOGY OF ILLINOIS. 



l y generally, however, so worn down over the outer half as to 

 present the appearance of a gradual slope, descending into the 

 shallow median depression, which is bordered on the opposite side 

 by the narrow wing-expansion. A mature tooth measures in length 

 along the anterior lateral border 18 mm. ; breadth across inner 

 margin, 9 mm. ; greatest depth of antero-lateral border, 2 mm. 



Mandibular posterior teeth triangular in outline, rather strongly 

 arched and inrolled at the outer extremity; antero-lateral border 

 steeply beveled from the prominent angulation of the coronal border, 

 a very narrow belt of enamel enveloping the upper edge, beneath 

 which the deep basal belt is more or less distinctly fluted in the 

 direction of inrollment, and terminating inferiorly in the relatively 

 short, attenuated edge, which constitutes about half the extent of 

 this side of the tooth; inner margin forming nearly a right angle 

 with the anterior border, which it joins in a rounded angle, slightly 

 curved sigmoidally in passing to the produced posterior angle of 

 the alate expansion, in breadth about equal to the length of the 

 anterior border of the tooth; postero-lateral border rapidly converg- 

 ing toward the outer extremity at an angle of 50 with the anterior 

 border of the crown, or about 60 with the inferior edge of the base 

 of that side, slightly arched and bounded along the upper edge by 

 the narrow enamel fold, the nearly vertical, slightly channeled basal 

 border posteriorly expanding beyond the coronal limits and produced 

 into a spur-like process forming the extreme angle of the postero 

 alation. The median portion of the crown presents a broad depres- 

 sion, denned on the one hand by the abrupt declivity descending 

 from the elevated angular ridge of the anterior prominence, and on 

 the other by the more gentle acclivity rising to the postero-lateral 

 border of the broadly expanded wing, presenting, as in the pre- 

 viously described form, extreme simplicity in coronal contour. Length 

 along the anterior border of the crown of a large example, 16 mm. ; 

 greatest depth of the antero-lateral slope, 5.5 mm. ; breadth at the 

 inner margin, 13 mm. 



The surface structure of both of the forms described above is 

 essentially identical. The punctae exhibit in a marked degree the 

 transverse arrangement in more or less regularly parallel lines co- 

 inciding with the curvature of the inner margin. This feature is 

 especially noticeable in the region of the coronal depression and the 

 less exposed inner portion of the crown, where, in specimens which 

 have undergone comparatively slight abrasion from trituration, the 

 surface presents a beautifully sculptured appearance from the slightly 



