58 PALAEONTOLOGY OF ILLINOIS. 



that between the fishes bearing the narrow, the broad, the sagittate, the oval, 

 the linear and the quadrate teeth, included in the Petdlodont family, there were 

 at least as great differences as between Myliobates, ^Etobatis, Zygobatis and 

 Rhinoptera. 



It will be noticed that in the present species we have the linear form of many 

 of the polyrhizoid group, and a root which is only a mass of consolidated root- 

 lets, and which seems just ready to divide into distinct radicles, and yet its 

 section presents the sharpest cutting edge and the most angular outline of any 

 of the series. It thus forms a connecting link between its most dissimilar asso- 

 ciates and illustrates the difficulty of separating them. 



Figures 18, 18 a and 18 b represent the posterior face, a part of the anterior 

 aspect and a central profile section — all natural size. 



Formation and locality: Burlington limestone, Burlington, Iowa. 



Chomatodus lomformis, N. and W. 



PI. Ill, Figs. 19, 19 a. 



Teeth very broad, thin, strap-shaped, with parallel margins 

 and' rounded at tke extremities; cutting edge straight through- 

 out the greater part of its length, curved downward at either 

 side, sub-acute, porous ; posterior face of crown comprising two- 

 thirds the entire height of tooth, slightly curved vertically, 

 straight laterally, smooth and polished; posterior coronal im- 

 bricating folds about 4, low but distinct, the upper ones widest, 

 forming a band half the height of the crown face; anterior face 

 half the height of the posterior, to which it is inclined at an 

 angle of about 45°, straight or slightly concave vertically; 

 coronal ridge a sharp angle, without distinct folds of enamel; 

 base occupying more than two-thirds of the anterior surface of 

 the tooth, slightly concave vertically; root nearly as broad as 

 the tooth, very short, extending but little below the posterior 

 basal folds, rounded in profile, roughened, not divided. 



The only specimen of this species which exists in the collection is broken at 

 one end, and does not therefore give the entire breadth. The height is 4J lines. 

 The breadth of the fragment, 17 lines. The total breadth could not have been 

 much less than 2 inches. 



