VERTEBRATES. 83 



The undulation of the enameled surface constitutes the most distinctive 

 character of the species. It resembles that of a species of Poecilodus, in the 

 collection, and they were perhaps once associated together. 



Figures 8, 8 a, 8h, represent the view from above and behind, and a profile 

 section, natural size. 



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



Helodus angulatits, N. and W. 



PI. V, Figs. 9, 9 a, 9 b, and 10, 19 a. 



Teeth elongated, linear in outline, obliquely truncated at 

 the ends, often sigmoidly curved, arched laterally with a mesial 

 ridge, but with no distinct tubercle or cone. In some speci- 

 mens the triturating surface on either side of the ridge is trans- 

 versely concave, in others one convex, usually smooth and 

 polished, marked with large, elongated pores; the root is usu- 

 ally oblique, short and wedge-shaped; length, half an inch to 

 an inch; breadth, 4 lines. 



These teeth are closely allied to H. Umax, but the numerous specimens in 

 the collection seem to have a character in common, which that species wants, 

 viz : their angular section. The ends are also generally truncated, not round- 

 ed, and they are more twisted. As has been before remarked, this group of 

 teeth are strikingly like some species of Stroplwdns y and it is a suggestive fact 

 that their microscopic structure diverges from that of their congeners in the 

 Carboniferous limestone in the same direction. The pores of the enameled 

 surface forming elongated and irregular lacunae, approaching in character the 

 angular reticulation of Stropliodus. 



Figures 9, 9 a, 9 6, represent the top, side views and sections of two of the 

 larger teeth ; figures 10-15, smaller specimens, apparently not specifically dif- 

 ferent. * 



Formation and locality: Burlington limestone, Quincy, Illinois. 



Helodus sulcatus, N. and W. 



PI. V, Pig. 16, 16 a. 



Teeth of medium or small size, thin; crown linear in outline, 

 broadest in the middle, the extremities truncated or somewhat 



