24 VERTEBRATA OF THE TERTIARY. 



than it is at the other Hmit of its existence, viz, the period of its disappear- 

 ance. 



For these reasons I must regard tlie latter criterion as the true one in 

 the discrimination of the subdivisions of geologic time, while the point of 

 the appearance of types must be looked upon as of provisional use only, 

 and this quite independently of the changes which discovery will from time 

 to time compel us to make in our knowledge of the distribution of life in 

 time and space. It must, however, be borne in mind that disappearance 

 may be due to two causes : first, to extinction ; and secondly, to modifica- 

 tion ; a distinction which is entirely essential. The case of disappearance 

 by modification is identical with that of appearance by modification, and 

 cannot be used otherwise in classification. It is, then, the period of extinc- 

 tion of types to which I have reference. 



With these principles in view, we attempt the comparison of the ex- 

 tinct faunae of Europe and North America, employing principally the 

 nomenclature of D'Orbigny for the former, and Hayden for the latter, in 

 the Mesozoic and Tertiary series. 



It is well known that no remains of Vertebrata have yet been discovered 

 in North America in strata of Silurian age, while several species are known 

 from the Upper Silurian of Europe. The latter are Placodermi and sharks, 

 and are not very numerous in species. They have been derived from 

 England, Gennany, and Russia. In America, the first fishes appear in the 

 Comiferous limestone at the base of the Devonian. Professor Newberry, 

 who has devoted much attention to this department, points out important 

 distinctions as existing between the Devonian fish faunae of North America 

 and Europe, and also to important coincidences. The first of these is the 

 occurrence of the genus Macropetalichthys in both continents; in Germany 

 in the Eifel limestone, and in America in the Corniferous limestone of Ohio. 

 The other examples are furnished by the Catskill beds of New York and 

 Pennsylvania, which contain a part of the fauna of the old Red Sandstone 

 of Scotland, including the genera Holoptychius and Bothriolepis* 



The structure of the Batrachia of the Coal-Measures is not yet suffi- 

 ciently well known to enable the most exact comparisous to be made, but 



• Oeological Survey of Ohio, i, pp. 264, 271. 



