On the Fossil Fishes of the Carboniferous Limestone Series of Great Britain. 401 
highest at the base, where they terminate adruptly ; osseous base a little wider 
than the crown. Diameter of base three or four lines.” —(M‘Coy.) 
The specimens forming the subjects of this description are in the Woodwardian 
Museum at Cambridge. They were obtained from the Mountain Limestone of 
Derbyshire and are stated by Professor M‘Coy to be tolerably abundant in that 
locality. I have not seen specimens in other collections or from other localities. 
Two species have been found in the Coal Measures of Belleville, Illinois (“ Geol. 
Illinois,” Vol. II., p. 70, e. seg.) ; one of very large size from the Burlington Lime- 
stone of Iowa, in America (“ Geol. of Illinois,” Vol. IV., p. 369). Specimens were 
discovered in the Devonian rocks by M. Trautschold, who named them Ostinaspis 
and considered that they were dermal tubercles, allied to those of the Rays; and 
M. de Koninck has described a species from the Mountain Limestone of the neigh- 
bourhood of Tournai, on the borders of Belgium (“ Fauna du Cale. Carb. de la 
Belgique.” p. 37). 
Prof. M‘Coy considered that the teeth of Petrodus approached more nearly in 
microscopic structure to those of the living Cestracion than any others, and this 
opinion appears to be shared by M. de. Koninck, with regard to the Belgian 
specimens. Messrs. Newberry and Worthen, who have described the American forms, 
in accordance with a suggestion made by Prof. Agassiz, consider that these bodies 
were not teeth, but the dermal tubercles of a fish probably resembling some of the 
living Rays or of the Squalaraja polyspondyla figured by Prof. Agassiz from the 
Lias of Lyme Regis (“ Poiss. Foss.,” Vol. III., pl. 48). The reasons on which this 
supposition is based are, that the base is very thin and very flat, much more so 
than in any Cestracionts, and too much so for the requisite solidity and stability of 
teeth of which the function was the crushing of resistant substances. ‘The base is 
also laminated on every side forming a thin, sharp, finely crenulated edge, which 
would prevent the teeth being in contact with each other, and in this respect offer a 
strong contrast to other fossil forms of Cestracionts. 
The teeth of Petrodus bear a close resemblance to those of Pleurodus from the 
Coal Measures. The latter are thin, the base is very small, and deeply concave in 
some instances, the edges are pectinated, and the surface is deeply ridged ; in all 
these respects bearing a close analogy with the teeth of Petrodus; and as the Pleu- 
rodus is undoubtedly a tooth, it may be assumed with considerable certainty that 
Petrodus is a tooth also. 
Genus—Ramphodus, Davis. 
Teeth, medium size, laterally, triangularly elongated ; lateral prolongations 
unequal. Crown acuminate, recurved posteriorly; antero-posterior diameter, 
small; coronal surface minutely punctate. Tooth bifurcates at right angles, one- 
fourth its greatest length from the apex ; base small. 
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