20 PALAEONTOLOGY OF ILLINOIS. 



other species have beeft frequently met with in Ohio, it is probable that they 

 will be hereafter found to occur in some localities in this State in considerable 

 abundance. The scale before us is less quadrate in form than those of Ohio, 

 and apparently represents a species up to the present time undescribed. 



Formation and locality ; In concretions of iron ore of the Coal Measures at 

 Morris, Illinois. 



Genus CLADODUS, Ag. 



Gen. Char. — "Teeth with broad, horizontal, semi-circular, 

 thick, bony, coarsely-fibrous base, rounded behind, truncated 

 in front; crown divided into long, sharp, subulate, conical 

 points, arranged along the straight truncated edge of the base; 

 medial cone much larger than the secondary ones, of which 

 latter the external cones are largest; all the cones striated 

 longitudinally, and either circular in section or with simple 

 cutting edges, slightly compressed." 



Cladodus robustus, N. and W. 



PL 1, Figs. 1, la. 



Teeth small, robust, as high as broad; base trapezoidal, as 

 long as median cone, moderately thick, arched before, widest 

 behind, where it is somewhat rounder and emarginate, smooth 

 throughout; median cone conical, recurved, equally arched 

 before and behind with a lenticular section and cutting edges, 

 smooth or finely and obscurely striated ; lateral denticles rudi- 

 mentary and tubercular, two on either side, exterior pair largest. 



In its form, curvature and section the median cone of this species closely 

 resembles that of 0. micropus, but it is smooth or very faintly striated. The 

 base is also largef and more angular, and the lateral denticles are less developed. 

 It is quite possible that a larger amount of material would show that the two 

 species shade into each other, and represent the dentition of different parts of 

 the mouth in the fish; but until such connecting links are found, the differences 

 which have been enumerated will serve to distinguish them. 



