214 PALEONTOLOGY OF ILLINOIS. 



margin, showing that the formative nucleus was situated nearly 

 midway the anterior margin ; in excessively worn individuals, whether 

 from use or abrasion subsequent to dislodgement from the jaw, the 

 posterior margin presents a broad convexity between the posterior 

 lateral angles, markedly in contrast with the actual curvature in 

 perfect teeth. A small tooth measures in greatest diameter across 

 the posterior margin 29 millimetres, length along inner border about 

 16 mm. The perfect state of preservation of many of the specimens 

 afford a fine exhibition of the character of the inferior surface, and 

 which is applicable to all the species of the genus to greater or less 

 degree. Conforming in a general way to the coronal contour, it is 

 enveloped in a thin smooth layer of dense matter faintly striated 

 longitudinally, forming a low ridge or angulation about one-third 

 the distance from the inner border from which the surface regularly 

 slopes to the lateral borders, the tooth showing its maximum depth 

 at the posterior margin. 



Teeth supposed to have been opposed to the preceding form, or 

 pertaining to the mandible, are distinguished by their subquadran- 

 gular outline and relatively narrower transverse diameter compared 

 to the length. The anterior margin is broadly arched forward with 

 a slight concavity near the inner angle, which is slightly auriculate 

 and sharply rounded at the outer angle ; the posterior margin pre- 

 sents essentially the same outline, both margins faintly channeled, 

 the posterior one most strongly inbevejed, the anterior nearly ver- 

 tical. The inner border is nearly straight, making a faint angula- 

 tion a little posterior of the middle in medium-size teeth, a slight 

 articular facet occurring at the truncated posterior angle, the artic- 

 ular face faintly channeled and somewhat strongly inbeveled ; outer 

 border very gently arched and gradually converging anteriorly, basal 

 portion in small and medium-size teeth rather deeply channeled, in 

 larger specimens showing a thick rim extending considerably beyond 

 the coronal limits and terminating in a produced spur posteriorly. 

 The coronal surface is very gently arched longitudinally forward 

 from the rather steep declivity bordering the posterior margin, and 

 gently convex transversely, a narrow channel bordering the inner 

 and posterior sides, which, together with the anterior margin, are 

 defined by an abrupt angle, the coronal enamel forming a narrow 

 fold encircling the tooth ; along the exterior border the coronal sur- 

 face is ordinarily abruptly rounded to the angular demarkation 

 between crown and base, but in large teeth this angle is less promi- 

 nent, though the coronal limits are still well defined ; a slight longi- 



