VERTEBRATES. 89 



seen from above and from the side, the latter in profile. Figure 4 a, the ante- 

 rior face of the largest tooth. Figures 5, 5 a, another series, seen from above 

 and in profile. Figure 6, a side view of a series of very small teeth. All from 

 the same locality — natural size. 



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



Cochliodus nobilis, N. and W. 



PI. VII, Figs. 1-4. PI. VIII, Fig. 1. 



The specimens from which our knowledge of this magnificent species is 

 derived, apparently include nearly the entire dental series; they were, however, 

 most unfortunately, collected without sufficient care to preserve every fragment 

 of all the numerous teeth found lying in contact, and no note was taken of their 

 relative positions ; had both these particulars been carefully attended to, it is 

 probable we should have been able to make out the complete dentition of Coch- 

 liodus, and thus throw a flood of light upon the classification of the placoid 

 fishes of the Palaeozoic ages. As usually found, the teeth of cartilaginous 

 fishes are severed from their connections, and all traces of the jaws which sup- 

 ported them are lost. In the present exceptional instance, however, the jaws 

 themselves are preserved, and the teeth were in contact, though generally dis- 

 placed, most of them lying in a confused heap. 



Jaws. — Fragments only of the jaws are visible on the specimens contained 

 in the collection, and such as are quite insufficient for determining their form ; 

 they are now thin and flattened, and much distorted, showing they had little 

 firmness or rigidity, and were doubtless, for the most part, cartilaginous, though 

 it is possible, in part ossified. They do not show a true bony structure, but 

 exhibit on fracture a fine granular composition, such as we have before seen 

 accompanying the more distinctly bony portions of the remains of cartilaginous 

 fishes, indicating, perhaps, a cartilage through which were disseminated innu- 

 merable granules of ossific matter. 



The structure of these jaws is very different from that of the bones of the 

 ganoid fishes, which are not unfrequently met with in the Carboniferous rocks. 

 Judging from this evidence and the fact that the teeth of Cochliodus are almost 

 universally detached and scattered through the rocks containing them, with no 

 traces of the jaws on which they were once set, we are led to infer that Prof. 

 McCoy was in error in removing Cochliodus from the place assigned it by Prof. 

 Agassiz among the sharks, and classing it among the Pycnodonts. We may 

 say further, that the specimens before us afford no confirmation of the views 

 suggested by Prof. McCoy, that the succession of teeth was from below upward, 

 as in the Pycnodonts, rather than from behind forward, as in the sharks. 



—12 



