44 PALAEONTOLOGY OF ILLINOIS. 



as defined by Owen; but witb tbat exception tbere could scarcely be a more 

 perfect type. 



Of the figures given, figure 3 represents the posterior surface, natm-al size; 

 3a the same, enlarged to two diameters; 36 profile or section through the cen- 

 ter of tooth, also twice the size of nature. 



Formation and locality: Keokuk limestone, Warsaw, Illinois. 



Antliodus simplex, N. and W. 



PI. Ill, Figs. 4, 4 a. 



Teeth small, long-elliptical in outline, superior border arched, 

 acute, striated on the anterior side, dotted posteriorly; posterior 

 face, long-elliptical, smooth and porous throughout, slightly 

 arched virtically, laterally straight, bordered below by a narrow 

 and low coronal ridge; anterior surface half the height of pos- 

 terior, nearly straight vertically, and in the central portion 

 laterally, toward either side regularly arched, upper two-thirds 

 finely striated vertically; anterior basal ridge low, simple, nearly 

 straight; base occupying half the anterior aspect of the tooth, 

 slightly arched both ways, inclined at right angles with the 

 anterior face of crown, roughened in the place occupied by the 

 root in the species; root, none. 



The only specimen of this species in the collection is somewhat worn, but is 

 sufficient to indicate its entire distinctness from all others described. The 

 posterior face has been exposed to long wear, when on the jaw, and the normal 

 character of this surface can hardly be deduced from it. It is now regularly 

 dotted all over with the ends of the enamel tubes, precisely as in Psammodus. 

 As has been mentioned, all the Petalodonts, when much worn, present some- 

 thing of this appearance. In this species it is more marked, however, than in 

 any other, and evidently depends on a peculiarity in the microscopic structure 

 of the crown. The cutting edge is quite thin, and the porosity of its posterior 

 surface is occasioned by the uniform direction of the enamel tubes. These are 

 set parallel with the slope of the anterior face, marking that face with vertical 

 striae, but dotting the posterior face with their ends cut obliquely. 



Fig. 4 represents an anterior view of tooth ; Fig. 4a, a profile section — both 

 natural size. 



Formation and locality : Burlington limestone, Burlington, Iowa. 



