358 PALAEONTOLOGY OF ILLINOIS. 



lateral angles, bordered at base by a narrow, prominent, bow- 

 shaped band of about four closely approximated enamel folds ; 

 posterior crown face long-elliptical in outline, five lines high, 

 concave in both directions ; root strong but short, two-thirds 

 as broad as the crown ; on the posterior face one-third, ante- 

 rior face two-thirds the entire hight, divided into seven or 

 eight oblong, thick, tooth-like radicles. 



This well-marked species has, at first sight, much the aspect of P. maynus 

 (McCoy, Brit. Palseoz. Fossils), but is smaller in all its dimensions, much thin- 

 ner, with fewer and relatively stronger radicles. One of its most characteristic 

 features is the prominent ridge formed by the imbricating enamel folds at the 

 base of the anterior face. 



This, with other fish remains, was obtained by Prof. A. Litton, who has 

 kindly loaned it to us for description. 



formation and locality : St. Louis limestone; St. Louis, Mo. 



GENUS ORODUS. 

 ORODUS CORRUGATUS, N. and W. 



PI. iii, fig. 18, 18or. 



TEETH of various forms and sizes, forming many rows ; of 

 these the largest are strongly arched in both directions, the 

 crown of the arch forming a broad, massive eccentric cone, or 

 protuberance, which is without rugae, but is coarsely granulo- 

 punctate. 



The crown surfaces of all these teeth with the exception of the compara- 

 tively smooth cone summits of the largest are strongly and sharply corrugated 

 by a medial, longitudinal crest, and numerous pinnate lateral crests which run 

 down to and strongly crenulate the sides. 



The lateral crests are beaded or pectinated; the whole forming an elaborate 

 system of surface ornamentation. 



The smaller teeth vary in size from 10 lines to 4 lines in length, being three 

 times as long as wide, long-elliptical in outline, the larger ones highest near one 

 end, showing a tendency to form the eccentric crown-cones of the larger series. 



The beautiful group of teeth represented in our figure form one of the most 

 interesting specimens yet discovered of the dentition of the fishes of the Car- 



