42 PALEONTOLOGY OF ILLINOIS. 



bounded by prominent ridges, one formed by a thickening of 

 the superior border, the other the basal ridge. The latter is 

 slightly bow-shaped and faintly marked with parallel folds ; 

 base very smooth, slightly arched laterally ; root three-fourths 

 the breadth of crown, very short, scarcely reaching the lower 

 border of the posterior coronal ridge, rough and irregular, 

 somewhat divided into rudimentary radicles. 



This species resembles, in its general aspects, A. robustus, but is readily dis- 

 tinguishable from it by its thickened edge, its concave anterior face, broader 

 base, shorter root, etc. Its profile, nearly that of the ladies' bonnets now in 

 fashion (1860), has suggested the name given it. 



Figs. 1, la, lb represent both faces and profile, natural size. 



Formation and locality : Keokuk limestone, Nauvoo, Illinois. 



Antliodus politus, N. and W. 



PL III, Figs. 2, 2 a. 



Teeth small, thick, exceedingly hard and dense, elliptical in 

 form; superior border of crown regularly arched, truncated and 

 slightly furrowed; posterior surface smooth and polished, ellip- 

 tical in outline, concave vertically, straight laterally; coronal 

 folds numerous, broad, flat, evenly arched downward; anterior 

 surface lenticular, long-pointed laterally, one-half the height 

 of the posterior surface, straight or slightly curved vertically, 

 arched laterally; anterior coronal ridge sub-acute, narrow, 

 prominent, and marked with several fine, parallel, enamel 

 folds; base sub-elliptical in outline, smooth; root rudimentary, 

 forming a prominent tuberculated ridge as long as half the 

 breadth of the tooth, set in the base near its lower border; 

 under the lens the structure appears very dense and close 

 throughout, a few pores opening along the borders of the supe- 

 rior marginal sulcus. 



l o" 



This, and the closely allied A. sulcatus, are among the most interesting species 

 represented in the collection Their relations to some of the nearly rootless 



