Fossil Fishes. 39 



from which it is necessary to separate his Pleuronectes, that they 

 may be carried back to the preceding order. 



" The better to comprehend the general results which I propose 

 to present, it will be necessary to say a few words on the existing 

 species. 



" We are now acquainted with about 8000 species of fishes. Of 

 this number more than three fourths belong to two orders of that 

 class, the presence of which has not yet been discovered in forma- 

 tions older than the cretaceous one, viz. to the Cycloides and to the 

 Ctenoi'des ; so that, truly, there is nothing analogous to them in the 

 whole series of the secondary rocks, even to greensand ; whilst the 

 remaining fourth belongs to the orders Placoides and Ganoides, not at 

 all numerous now, but which existed alone, during the whole peri- 

 od which elapsed from the time the earth began to be inhabited, till 

 the moment in which the animals found in the greensand appeared. 

 The same precise pro])ortion existing in the orders of the class to 

 which attention is more especially solicited in this work, is a fact 

 which is truly remarkable ; it is almost inconceivable, but still un- 

 doubted, since it is a mere matter of calculation ; and moreover, we 

 may remark, it is not only in general that this regular arrangement of 

 the groups may be noticed ; but in each order, and in each family 

 even, the genera produce in their affinities analogous series, so that 

 the differences of organization become the distinctive characters for 

 the geological epochs, even in those species which are seen for the first 

 time. I can now state this result with confidence, after having re- 

 viewed the general conclusions to which I have arrived in the study 

 of fossils, and supported by the examination of 250 new species dis- 

 covered in British Collections, without having met with a single 

 exception in the 800 species with which I am now acquainted. 

 These essential organic differences have an especial reference to the 

 nature of the integuments, and to the mode in which the vertebral 

 column terminates in the caudal fin, in other words, to the relations 

 which subsist in the animal to the material world which surrounds 

 him, and the structure of that organ which is essential to his loco- 

 motion. I shall now very briefly point out those distinctions, and, 

 at a future time, will fully enumerate all the fishes belonging to each 

 great formation ; for in thus presenting a general description, it will 

 easily be understood that I cannot enter largely into detail. 



" That we may appreciate at its just value the study of fishes in 

 general, and of fossil fishes in particular, we ought never to lose sight 



