VERTEBRATES. 77 



The figures given represent the crown surface, posterior face and profile, 

 natural size. 



Formation and locality : Keokuk limestone, Warsaw, Illinois. 



Helodus biformis, N. and "W. 



PI. IV, Fig. 22, 22 a. 



Teeth small, in series of three or more, the central one being 

 largest and quite different in form from one of those associated 

 with it. It is oblong in outline, arched in both directions, its 

 highest point being nearly central, its anterior margin some- 

 what waved with a prominent central gibbosity; the anterior 

 tooth is nearly as large as the central, but narrower, somewhat 

 curved and adapted to the outline of the central one; its sur- 

 face is raised into three prominences, separated by shallow 

 grooves, of which the central one is somewhat the largest. 

 The small posterior? tooth is about half the size of the cen- 

 tral one with a similar median gibbosity. The enameled 

 surface of all these teeth is uniformly punctate, the termini of 

 the enamel tubes being raised above the general surface; the 

 roots are short, narrower than the crown, oblique. 



This very distinct and peculiar species exhibits characters considerably dif- 

 ferent from those of any other with which it is associated in the collection. 

 It is from a geological horizon which has furnished but one other species of 

 Hehdus, and it may hereafter prove to be a type of a distinct genus. There 

 are contained in the fragments of stone in which it is imbedded numerous small 

 kidney-shaped teeth, having precisely the same structure, and doubtless once 

 belonging to the same fish. We may conjecture that those which we have now 

 described formed a medial series, flanked on either side by broader and flatter 

 teeth, as in the living Cestracionini. 



Figure 22, the series of three teeth described above, natural size ; 22 a, a 

 large, detached tooth. 



Formation and locality: Kinderhook group, Burlington, Iowa. 



