418 PALEONTOLOGY OF ILLINOIS. 



acuminate. In large, mature teeth, the base presents a broad, thick, 

 rhomboidal plate, with more or less parallel, curved sides and sharply 

 marked angles, inferior surface in nearly the same horizontal plane as 

 the crown, faintly concave, smooth or striated, abruptly beveled and 

 coarsely roughened in the thickened face, the opposite face deeply chan- 

 neled and strongly defined from the anteriorly produced basal border 

 of the crown, in lateral diameter equal to and in breadth exceeded by 

 that of the crown. The crown forms a massive, rather low, broad, 

 moderately convex prominence, the greatest prominence of which 

 extends in an antero-posterior direction, and produced into a low, 

 obtuse, subcentral ridge, culminating above the abruptly sloping outer 

 face, declining in a broad gentle convexity to the opposite margin, 

 which is gently arched posteriorly and approximately parallel to the 

 anterior margin, both of which are more or less irregular or undulated 

 in outline, constricted along the base, one extremity abruptly truncated, 

 one of the angles obliquely so, the opposite end broadly rounded and 

 slightly narrowed ; the coronal surface is ornamented with numerous 

 delicate, slightly undulating plicae, which appear as flattened or sharp 

 thread-like lines over the body of the crown, obsolete in the region of 

 the obscurely defined crest, but towards the basal margins in either 

 face becoming more strongly marked and intricately interlaced, pro- 

 ducing the delicate reticulated ornamentation encircling the basal bor- 

 ders of the crown. 



Small teeth presenting essentially the same form and coronal features 

 noted above ; but the base is more obliquely produced and gradually 

 beveled to a sharp edge along the posterior margin, and the correspond- 

 ing channel of the opposite face is relatively broader. The crown is 

 elliptical in outline, extremities broadly rounded, the median prominence 

 less eccentric and comparatively more strongly developed, forming, as 

 in the large teeth, an obtuse ridge traversing the crown at right angles 

 to the obscurely defined, submedian crest; the coronal plicae are well- 

 marked, forming an elegantly wrought belt along the inferior borders, 

 precisely as in the large teeth, in both of which the crown is enveloped 

 in a polished enamel-like layer, except along the crest and median 

 prominence, where it has usually been abraded to greater or less extent, 

 exposing in the triturating surface a delicate punctate structure. 



By a remarkably uniform gradation the latter teeth pass into smaller 

 and more and more acuminate individuals, until we arrive at minute teeth 

 in which the median prominence is excessively developed, forming a 

 lofty, anteriorly flattened, posteriorly broadly arched, laterally deflected, 

 obtusely pointed cone, around the basal borders of which are clustered 

 a few relatively strong plicae; the base presents essentially the same 

 features previously observed, and the coronal ornamentation, traced 



