VERTEBRATES. 71 



This species is so much like P. pateMformds^ McCoy, {Pal-xozoic Fotssils, p. 

 637, pi. 3. G. figs 6, 7, 8,) that I have hesitated to consider it distinct. In 

 P. patelliformis, however, the radiating ridges of the crown are represented by 

 Prof. McCoy as being always strongly sulcated transversely, almost pectinated 

 in fact, while in the great number of specimens in the collection none exhibit 

 this character; a slight rugosity, rarely visible, is the only approach to it. 



Prof. Agassiz has suggested that these bodies were not teeth, but the dermal 

 tubercles of some of the ancient sharks. This supposition is rendered probable 

 by their resemblance to the cretaceous tubercles of living plagiostoinous fishes, 

 particularly the Kays, and by some peculiarities of their structure as compared 

 with other teeth. It will be seen that the base is exceedingly thin and very 

 flat, much more so than in the teeth of any of the cestracionts, and too much so, 

 we might suspect, for the requisite solidity and stability of teeth of which the 

 function was the crushing of resistant substances. The base is, also, not only 

 thin and weak, but laminated on every side in a thin, sharp, finely crenulated 

 edge. This shows that if teeth, they must have been isolated and nowhere in 

 contact with each other. In these respects they offer a strong contrast to 

 Orodus, Acrodus, Ptychodus, etc., to which they have otherwise considerable 

 resemblance. In all these genera the bases of the teeth are much thicker, and 

 of forms that prove them to have been in contact with each other, and even 

 matched together with a kind of pavement. 



The extreme probability of the suggestion of Prof. Agassiz will be at once 

 seen by reference to the figures which he gives of a fossil ray from the Lias of 

 Lyme Regis, (Pom. Foss. Atlas, Vol. 3, pi. 42, 43,) the dermal tubercles of 

 this fish (Squaloraja polyspondyld) exhibiting a marked resemblance to those 

 bodies called Petrodus by Prof. McCoy, and considered by him to be teeth. 



There is great variation in form and size in the large number of these fossils 

 included in the collection, some being ten times as large as others, some circu- 

 lar and some long-elliptical in outline, some being acute at summit and marked 

 with sharp divergent ridges, others nearly smooth, as though much worn. 

 With the exception, however, of that which formed the basis of the succeeding 

 specific description, they should probably be all regarded as the exuvte of the 

 same species of fish. A large, sub-triangular, nearly smooth specimen, from the 

 Coal Measures of La Salle, exhibits some peculiarities which may have a specific 

 value. Without further evidence, however, we should hardly be justified in 

 regarding it as distinct from those we have designated by the name of P. occi- 

 dentalis. 



Figures 15, 15a and 156, top and side views of two teeth; 16, 16a, another 

 tooth, perhaps specifically distinct. 



Formation and locality: Coal Measures, Belleville, Illinois. 



