VERTEBRATES. 53 



crown terminating above in a cutting edge which has an arched 

 outline, more or less undulate and porous ; posterior crown 

 face more than half the height of the tooth, slightly concave 

 vertically, arched laterally at either end; posterior basal folds 

 about 3, forming a flattened band, half the height of crown 

 face; anterior crown face a little less high than posterior, 

 nearly straight vertically and laterally, except at the extremi- 

 ties, where it is curved forward, smooth and polished, but under 

 the lens showing pores throughout; basal ridge narrow, dis- 

 tinct, straight, except at the ends, where it is slightly turned 

 upward, without imbricating folds; root very nearly as wide as 

 crown, less than half as high undivided, but marked along its 

 anterior surface by a series of deep and nearly uniform pits or 

 depressions, separated by narrow, prominent, vertical costae ; 

 toward either lateral extremity the root is gradually narrowed, 

 and here the costae are somewhat divergent from the centre. 



This species resembles the preceding one in many respects, but is readily 

 distinguishable by its thin, flat crown, curved at either end, and by its thin, 

 deeply pitted root. 



Figures 13, 13 a, represent the anterior face of an imperfect tooth, with a 

 profile section, of the she of nature. 



Formation and locality : Chester limestone, Chester, Illinois. 



Chomatodus pusillus, N. and W. 



PI. Ill, Figs. 14, 14 a, 14 6. 



Teeth small, slender, linear, broader than high ; posterior 

 face of crown half the entire height, linear or long-lenticular 

 in outline, straight laterally, concave vertically, cutting edge 

 sharp, fine'^ crenulated and more or less undulated, whole pos- 

 terior face striated vertically, striation stronger along the edge; 

 as in the typical Petalodonts; basal angle very prominent, 

 showing two or three narrow enamel folds ; anterior surface 

 equaling posterior in height, of similar outline, slightly arched 



