336 M- Asassiz on Fishes. 



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lower in the scale. In this respect, it is with fish nearly as it is with 

 the manimiferae and reptiles, whose species, in general very limited, 

 belong in the series of formations with little vertical distance, to genera 

 which are different, without passing insensibly from one formation to 

 another, as is often witnessed in certain shells. This is one of the 

 most interesting facts which I have observed, namely, that I do not 

 know a single species of fossil fish which is successively found in two 

 formations, whilst I know a great number which have a considerable 

 horizontal distribution. Besides, the class of fishes moreover presents 

 to zoological geology the immense advantage of extending across all 

 the formations, and* of presenting in a class of vertebral animals a 

 point of comparison regarding the differences which exhibit them- 

 selves in the longest lapse of known time, of animals constructed gene- 

 rally on the same plan — of animals of a class which already counts a very 

 great number of fossil species.for the most part referable to types which 

 exist no longer ; and whose affinities with the living species are as 

 distinctly marked as those which ally the Crinoidea to the ordinary 

 Echinodermata, the Nautiles, and the Sepia to the Belemnites, and 

 the Ammonites, the Pterodactyls, the Ichthyosaurae, and the Ple- 

 siosaurae to our Saurians ; and the living Pachydermata to those which 

 of old inhabited the borders of the lakes around Paris, or the plains 

 of Siberia. 



The fishes of the tertiary rocks, are those on which I the least 

 dilate, because they approach nearest to the living species, and be- 

 cause their study may be undertaken by means of works which are 

 already published upon Ichthyology. At the same time, considering 

 the prodigious number of living species to which they approximate, 

 it is often very difficult, in the condition in which they are discover- 

 ed, to identify them, or rather exactly to appreciate their distinctive 

 characters. 1 will only remark, in general, that up to the pre- 

 sent moment, I have not found a single species which is perfectly 

 identical with those of our seas, except that little fish which is found 

 in Greenland, in the geodian clay, and the geological age of which is 

 unknown to me. 



The species of the Crag of Norfolk, of the superior subapennine 

 formation, and of the molasse formation, for the most part approach 

 to the common genera of tropical seas ; such as the Platax. the Great 

 Carcharias, the Great Myliobates, &c. 



In the inferior tertiary formations, — in the London clay, the coarse 

 limestone of Paris, and of Monte-Bolca, we find that at least a third 

 of the species already belong to genera which no longer exist. In the 

 comparative tables of ail known fishes which I propose soon to pub- 

 lish, I shall give the names of all the fossil genera and species of all 

 geological epochs, and at the same time shall point out all the locali- 

 ties in which they are found, and the corresponding genera of the 

 actual existing creation, in a separate column. 



The Chalk group includes more than two-thirds of those spe- 

 cies which are referable to genera which have entirely disappeared ; 

 and here we already see some of those singular forms which prevail 

 in the oolite series. At the same time, as a whole, the fishes of the 

 chalk formation much more resemble the general character of the 

 fishes of the tertiary series than that of those belonging to the oolite 

 group ; and this to such an extent, that did I regard only the fishes, 



