VERTEBRATES. 91 



relative position of each form was. It seems probable, however, that the 

 smaller conical teeth formed several series intermediate between the larger and 

 broader ones, upon the symphysis of the jaw. In the living Cestracion we find 

 a precisely similar arrangement. The rami of the jaws are covered with a 

 series of broad, flattened plates, fitted for crushing only, while the mesial por- 

 tion of each jaw is occupied by numerous rows of small, pointed teeth, dimin- 

 ishing in size from front to rear. The smaller teeth in the group before us ' 

 would, as has been remarked, if taken by themselves, be referred to Uelodus, 

 and it is highly probable that a large number of the species of Uelodus now or 

 heretofore described, were once associated with teeth of very different form, 

 some of which may be known under other names. Several rows of teeth, 

 forming a regular gradation of size, generally similar to those under consider- 

 ation, have been discovered in the old world, and are described in the works of 

 Agassiz, McCoy, etc. Prof. McCoy, in speaking of a series of this kind, 

 referred to Uelodus Isevissimus, Agassiz, {Brit. Palseoz., Foss., p. 631), says, 

 "they diminish in size from behind forward." This seems to us improbable, 

 as upon the supposition that they occupied the mesial space between the larger 

 convoluted teeth, that space would be broadest anteriorly ; aside from this we 

 have the analagous arrangement of the teeth in both the Cestracionts and Hyho- 

 donts, as well as in all the living genera of true sharks, in confirmation of the 

 view that the teeth in these rows diminished in size from the front rank back- ' ' • 

 ward. 



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



Cochliodus? crassus, N. and W. = Sc^.tvLrK^L 



i yv i 



PI. VIII, Figs. 2, 2 a. 



Teeth thick, massive, sub-quadrate in outline, the two loilger 

 sides respectively convex and concave, one of the lateral angles 

 of the concave side being prolonged, the superior surface is 

 strongly arched from the front backward with a lateral sulcus 

 extending to the produced posterior angle; it is highly polish- 

 ed, marked with faint waved lines of growth parallel with the 

 convex anterior margin, and a few obscure wrinkles crossing 

 these. The central portion of the tooth is evenly punctate ; 

 near the margin the enamel tubes being concealed by an 

 exterior coating, as is frequently the case with teeth of this 



