26 PALAEONTOLOGY OF ILLINOIS. 



striaj. From each of its congeners 0. Zygopus differs in several important 

 characters. The basal sinus is deeper than in any other species known. The 

 striation of the surface is finer and more crowded than in 0. gracilis or C. cos- 

 tatits. The lateral denticles not as long and slender as in 0. gracilis, nor as 

 broad and flat as in C. costatus. In C. Hibberti the lateral cusps are much 

 more unequal, the median cone less broad at base, and with a very shallow 

 sinus. 



Figures 9 and 9 a, anterior face and base of small specimen; 10, anterior 

 face of a larger tooth ; all natural size. 



Formation and locality: Chester limestone, Chester, Illinois. 



Cladodus ferox, N. and W. 



PI. I, Fig. 11 and 11 a. 



Teeth of large size, breadth greater than height; base semi- 

 elliptical, with a shallow sinus in front, smooth throughout; 

 median cone slightly curved backward, conical, with a circular 

 section, finely and evenly striated longitudinally ; lateral den- 

 ticles about 5 on either side ; exterior pair one-third the height 

 of the principal cone, conical, acute, striated throughout, diver- 

 gent from median cone, and strongly curved backward ; inter- 

 mediate denticles about 4 on either side, largest, half the size of 

 exterior pair, conical, striated, projecting forward. 



The specimens of this species, contained in the collection, do not fully give 

 the form of the principal cone, but it is apparently slender, conical, and acute. 

 The lateral denticles are of the same character, but curved backward at very 

 different angles ; the posterior pair being set on the same line with the median 

 cone, and hooked backward much more than that, while the intermediate cones 

 are set in advance of these, and less curved. Bristling, as these teeth did, 

 with a multitude of sharp, fang-like denticles, pointing in different directions, 

 they must have been among the most effective of all the varied forms of dental 

 armament possessed by plagiostomous fishes. There is no described species 

 with which this is liable to be confounded. The form of the principal cone 

 was similar to that of C. striatus, Ag., (Pois. Foss. Vol. 3, _p. 197, Atlas, Vol. 

 3, tab. 22 b, Jigs. 14-17), but the striation of the surface was finer, and the 

 number and form of the lateral denticles quite different. 



Among those associated with it in the collection, perhaps Cladodus spinosus, 



