56 PALAEONTOLOGY OF ILLINOIS. 



Figures 16 and 16a represent the posterior aspect of a broken tooth and a 

 section, natural size. 



Formation and locality: Coal Measures, LaSalle, Illinois. 



Chomatodus molaris, N. and W. 



PI. Ill, Figs. IT, 17a. 



Teeth thick and strong, broader than high, outline linear or 

 oblong, with nearly parallel sides ; posterior face of crown half 

 the entire height, concave vertically, straight laterally, smooth; 

 posterior basal folds 2-3, forming a prominent band or ridge 

 nearly half the height of the crown face ; anterior surface nar- 

 rower than posterior, -forming an arched summit, of which the 

 posterior margin is scarcely higher than the anterior; in worn 

 specimens it is lower; along the anterior border of this surface 

 is a shallow sulcus, dividing off a narrow line, which forms the 

 representative of the usual basal folds. The triturating surface 

 formed by the homologue of the anterior crown face, when 

 much worn, shows a few large pores, but in preparation for the 

 severe service to which they were exposed, the entire teeth 

 were exceedingly close and compact in texture, and the sur- 

 faces are all smooth and polished. The base is concave verti- 

 cally, straight laterally, strongly inclined below toward the 

 posterior face, giving a V-shaped section to this part of the 

 tooth. The root is strong but short, reaching little below the 

 line of the posterior basal folds. 



This may be regarded as the type of a small group of species, represented in 

 the collection by a large number of specimens, most of which are, however, 

 too imperfect for description. They exhibit the pctalodont type of structure 

 throughout, but in their laterally elongated form, their truncated summits, they 

 approach very nearly to the teeth of some of the rays, and it is evident that 

 their functions were the same as those of the teeth of the Myliobatini, i. e. they 

 were employed in crushing shells, corals and other resistant substances, for 

 which both strength of form and great hardness were required. 



