86 PALAEONTOLOGY OF ILLINOIS. 



.somewhat oblique, narrow, linear, section quadrangular, slightly 

 .shorter than the crown. 



But a single, though quite perfect specimen represents this species in the 

 collection. It is a handsome, highly ornamented tooth, though its affinities are 

 somewhat obscure. We have referred it,' with some doubt, to Ghomatodus, 

 though it is by no means certain that it will not be hereafter considered generi- 

 oally new. • 



Figures 17, 17 a represent the crown face and section, natural size. 



Formation and locality : Keokuk limestone, Nauvoo, Illinois. 



Chomatodus elegans, N. and W. 



PI. V, Figs. 18, 18a, 18 6. 



Teeth of large size, laterally elongated ; crown linear or long- 

 elliptical in outline, somewhat curved, having a prominent crest 

 running along the medial line, which, near the central point, 

 swells into a rounded tubercle or gibbosity. This mesial crest 

 is on one side arched, on the other side concave, being bordered 

 by a deep and broad furrow, which runs the entire length of 

 the tooth. The surface of this ridge is deeply and regularly 

 pitted by elongated pores or depressions, which have a trans- 

 verse direction. On either side of the central gibbosity they 

 are larger, giving to the ridge the crenulated and striated 

 appearance of the cutting edge of some species of Petalodus. 

 The base of the crown is ornamented by numerous imbricating 

 folds of enamel, those of the posterior descending lower than 

 those on the anterior surface — the former being more numerous 

 and finer; the anterior edge of the base of the crown is also 

 prettily crenulated. The root is in height nearly equal to the 

 crown, its anterior face nearly straight, posterior oblique. 

 Length, 1 inch, 7 lines; breadth, 6 lines; height, 8 lines. 



This is one of the handsomest species of the genus. In the specimen before 

 us the medial crest is nearly white, while the pits upon its surface contain, at 

 bottom, black enamel, the basal folds on either side are also jet black. This 



