80 PALAEONTOLOGY OF ILLINOIS. 



gibbosity protrudes opposite the highest point; base large and 

 thick, as high and wide as the crown, coarse and porous, one 

 face slightly arched outward and striated vertically, the oppo- 

 site one concave, under surface oblique, smooth. Length, 1 

 inch; height, 7 lines. 



In the character of the root, and the porosity of the crown, this tooth shows 

 some resemblance to H. denticulatus, and it is possible they are parts of the 

 same dental series ; there are, however, very marked differences in the form of 

 the crown, and, as we have no evidence of identity, we are compelled, for the 

 present, to regard them as distinct. 



Figures 3, 3 a, represent side view and section, natural size. 



Formation and locality: Keokuk limestone, Hamilton and Warsaw, Illinois. 



Helodus placenta, N. and W. 



PL V, Fig. 4, 4 a. 



Teeth of medium or small size, sub-elliptical in outline, flat- 

 tened, thin; crown slightly and irregularly arched in both 

 directions, finely porous throughout, thicker on the posterior 

 than the anterior edge, margin somewhat rounded, root oblique, 

 presenting a vertical face as high as the crown on one side of 

 the tooth, its inferior surface sloping up to the opposite border ; 

 the vertical side of the base marked with strong, irregular, 

 vertical ridges. 



The tooth is somewhat like Helodus planus, Ag , but is less angular in out- 

 line, more finely punctate, and wants its characteristic crenulated margin. 



Figures 4, 4 a, are intended to represent the appearance of one of these 

 teeth, when viewed from above and from the posterior side. 



Formation and locality: Kinderhook group, Burlington, Iowa. 



Helodus limax, N. and W. 



PI. V, Fig. 5, 5 a, 5 b. 



Teeth of medium size, laterally elongated, somewhat curved, 

 extremities rounded ; entire crown smooth and highly polished, 



