52 TAL.EONTOLOGT OF ILLINOIS. 



and lower borders being straight and parallel throughout most 

 of the breadth, but curved to meet at each end. The surface 

 slightly concave and finely striated vertically, straight later- 

 ally; the upper margin showing a few relatively large pores ; 

 basal folds 3-4, forming a flattened band little more than half 

 as high as the crown face; anterior surface nearly horizontal, 

 forming a flat top to the tooth. This is a little narrower than 

 the posterior crown face, lower before than behind, slightly 

 concave in an antero-posterior direction; base broader than the 

 anterior face, flat ; root nearly as broad as the crown, very 

 short, projecting but little below the posterior imbricated folds, 

 more or less waved in outline, but scarcely divided. 



This species bears considerable resemblance to C. loriformis in its general 

 aspect, but is relatively broader and lower, has the crown more rounded at the 

 ends, has a more nearly horizontal summit when viewed in profile, etc. It 

 would seem also that some of the teeth described by Prof. Agassiz. (Poiss. 

 Pass. Vol. 3, p. 108, tab. 12, fig. 5-12,) under the name of Chomatodus 

 linearis were of similar form. There are probably several distinct species, 

 however, included in the series represented by Prof. A., part of which should 

 be included in Antliodus — as Fig. 5, which is considerably like our A. parvidus, 

 and others perhaps in Helodus. The specimen represented by Fig. 6, (loc. cit.} 

 evidently represents a species closely allied to ours, though the descriptions of 

 C. linearis, by Agassiz and McCoy, prove it different. These represent the 

 crown as surrounded by a line of basal folds, while in our specimens of P.gra- 

 cillimus, as in all the group to which this species belongs the imbricated folds 

 are scarcely, if at all, visible, except on the posterior face. The profile section 

 of Chomatodus linearis is also quite different, the superior margin being much 

 sharper. 



Figures 12 and 12 a represent the posterior aspect and section, natural size. 



Formation and locality: Burlington limestone, Burlington, Iowa. 



Chomatodus cultellus, N. and W. 



PI. Ill, Fig. 13, 13 a. 



Teeth transversely elongate, thin and knife-like; central 

 portions straight and flat, the ends strongly curved forward; 



