230 PALAEONTOLOGY OF ILLINOIS. 



tance ; near the borders the surface is depressed and denned from 

 the projecting basal rim by a delicate fold, the enamel terminating 

 in an acute angle at the postero-lateral extremities. The surface is 

 usually worn, exposing the minute, irregular punctae, the lateral 

 edges and inner margin retaining greater or less areas, enveloped 

 in the superficial enamel layer, which shows exceedingly delicate 

 rugse radiating towards the lateral and posterior sides. Inferior 

 surface corresponding in longitudinal concavity to the convexity of 

 the crown, transversely gently arched with a slight median angula- 

 lation, the dense external layer smooth or faintly fluted longitudin- 

 ally. A large tooth measures in greatest transverse diameter between 

 the posterior cornua 20 millimetres ; the enameled coronal area 

 shows a breadth of 13.5 mm. ; length of tooth along the median 

 line 15 mm. ; depth at the posterior margin 4 mm. 



Teeth, referable to the mandible of the same species that bore the 

 preceding form, which they resemble in outline and coronal aspect, 

 are specially distinguished by their relatively greater breadth com- 

 pared with the length, and rather strongly rolled contour of the 

 crown from behind forwards. The anterior margin is somewhat 

 strongly arched forward from the slight auriculate antero-lateral 

 angles, meeting in a rounded angulation at the median line, the in- 

 beveled coronal portion forming a well-defined narrow belt, beneath 

 which extends the deep border of the slightly channeled basal por- 

 tion, in which particulars also there is unmistakable identity with 

 the preceding form ; the lateral borders gradually converge, making 

 obtuse angles in front and posteriorly produced, the basal rim form- 

 ing a conspicuous border, slightly upraised along the outer side, 

 rounded below and merged into the inferior surface; posterior mar- 

 gin unknown probably approximating the curvature of the opposite 

 extremity. Coronal surface strongly arched longitudinally, the trans- 

 verse convexity moderate and regular, with a shallow, narrow de- 

 pression parallel with the lateral borders, along which an exceedingly 

 delicate fold defines the crown from the basal rim; in front the 

 surface is worn into a lunate depression of greater or less extent 

 and depth, the punctae and the delicate rugose ornamentation of the 

 enamel layer agreeing with the surface appearances noted in the 

 opposed form; towards the posterior margin the surface shows dis- 

 tinct lines of growth, making a broad arch backward, and deflected 

 on nearing the lateral borders, but which does not necessarily indi- 

 cate the outline of the posterior margin, which doubtless in the 

 perfect state presents a concavity corresponding to the convex out- 



