VERTEBRATES. 55 



and at first sight will perhaps not be distinguished from it; closer inspection 

 will, however, enable one to separate them. In C. affinis the posterior imbri- 

 cating folds are fewer, forming a' narrower band which is not so distinctly 

 turned up at the ends. The base, too, is very much more concave. 



Figure 15 and 15 «, represent the posterior aspect and section of an imper- 

 fect tooth, natural size. 



Formation and locality: Keokuk limestone, Appanoose, Illinois. 



Chomatodus angularis, N. and W. 



PI. Ill, Figs. 1G, 16a. 



Teeth laterally elongate, broader than high, oblong in out- 

 line, thick and strong; posterior crown face little more than 

 half the height of the tooth; linear in outline, the superior 

 border straight, forming an obtuse or truncated triturating 

 edge, concave vertically, straight laterally, smooth below, 

 porous above; posterior basal folds about ten, the upper much 

 the broadest, forming a flattened band nearly as high as the 

 crown face. These folds are straight throughout all the middle 

 portion of the tooth, but are suddenly turned up at either end 

 toward the straight upper margin; anterior crown face linear, 

 half the height of the posterior surface, toward which it is 

 inclined at an angle of about 60°, concave vertically, straight 

 laterally; anterior basal folds about three, in a flattened ridge 

 half the height of the crown face ; base nearly flat and smooth, 

 forming a plane at right angles with the anterior crown face; 

 on this plane the rudimentary root is placed as a prominent 

 ridge, not reaching down to the lower line of the posterior basal 

 folds. 



Though closely allied to the succeeding species, C. molaris, this exhibits very 

 important and distinctive differences. It is more angular in all its parts. The 

 root is smaller, the posterior basal folds much more numerous, the angle inclu- 

 ded between the crown faces different, and the triturating surface is formed by 

 this truncated angle, rather than by the arched anterior crown surface. Its 

 position on the jaw must therefore have been different, and its broader surfaces 

 more nearly horizontal. 



