332 



FOSSIL ICHTHYOLOGY. 



Fossil Of all the vertebrated animal creation fishes are by far 

 Ichthy. ffig most numerous species, and are the most widely dis- 



°'°gy- tributed in the strata composing the crust of our earth. 

 ^^/""'^ Some are found in formations as old as the Silurian, and 

 become very numerous in all the more recent formations, 

 even to the Tertiaries containing the remains of mammals. 

 Fossil fishes, therefore, possess a high geological interest ; 

 and aid us greatly in determining the successive changes 

 which liave affected the surface of our planet. 



Our knowledge of Fossil Fishes was, imtil very lately, 

 exceedingly imperfect. Indeed, with the exception of 

 the Iltilithologia Veroiiese, published in iTt'G, we had 

 scarcely a tolerable attempt at the determination of fossil 

 species, before the appearance of the magnificent and truly 

 scientific Recherc/ies sur les Poissons Fossiles of M. 

 Agassiz (1833-43). In the first-mentioned work, however, 

 fossil species are often confounded with fishes now existing; 

 although not a single species of the immense accumulation 

 in Monte Bolca, though evidently belonging to a compara- 

 tively recent geological epoch, is identical with any fish 

 now existing. Good descriptions, it is true, of some fossil 

 fishes were scattered through various works ; but they 

 were connected by no general classification to unite them 

 into allied groups, or to exhibit their relations to the 

 various geological series of formations that compose the 

 crust of our earth. But the work of Agassiz was a gigantic 

 undertaking ; and the success which has crowned his 

 labours is most creditable to the unwearied industry and 

 great talents of this distinguished naturalist. He has pre- 

 sented his readers with full descriptions or notices of about 

 1000 species of fossil fishes, illustrated by accurate and 

 exquisite designs, and supplied us with a method of deter- 

 mining the species that may be hereafter discovered, with a 

 certainty and precision that has no parallel in this branch 

 of natural history. This is the result of the examination of 

 more than 20,000 specimens of fossil fishes preserved in the 

 public and private museums of Europe ; and now that he 

 has taken up his abode in North America, we know that 

 he pursues this subject with his accustomed energy, and will 

 no doubt add greatly to our knowledge of this subject, one 

 of the most interesting branches of geological paleeontology. 

 In his work on the Fresh Water Fishes of Europe, ]M. 



Agassiz had pointed out the important character to be 

 obtained from observing the form and structure of the scales 

 of fishes ; and this character he successfijlly employs as the 

 distinction of the four orders into which he divides the class 

 of fossil fishes. He had before pointed out the constancy 

 of the structure of the scales in each kind of fish ; a charac- 

 ter which had been too much overlooked by preceding 

 naturalists, although this integument is a principal medium 

 by which external influences reach the internal organization 

 of the animal. He considers it as a sort of external skele- 

 ton, differing widely and invariably in its structure, in each 

 of the great divisions of this class of animals. The scales 

 have, too, a peculiar value in fossil ichthyology, from the 

 perfect manner in which their durable nature has preserved 

 their minutest features in their stony repositories. Thus, 

 the examination of a few scales determines the order to 

 which the specimen should be referred, and often points out 

 their affinities to existing species. 



The orders are divided into Families, chiefly from the 

 structure and position of the fins, from the form of the bones 

 of the head, especially of the teeth, and the structure of the 

 operciila or gill covers ; and a most important character is 

 obtained, especially in the Placoid order, from the spinous 

 ravs of the several fins, which often are tiie formidable de- 

 fensive weapons of the fish. 



In these researches, M. Agassiz has freely acknowledged 

 the assistance he has received from contemporary naturalists, 

 and speaks in high terms of the valuable labours of Blain- 

 ville, Eichwald, Buckland, Sedgwick, Murchison, Miller, 

 Hibbert, and especially of Richard Owen, whose most 

 valuable Odontography has thrown so much light on 

 zoological subjects. 



In the following sketch of this new and interesting subject 

 we shall endeavour to present, as far as the necessary limits of 

 the work will permit, a general viewof the system of Agassiz. 



M. Agassiz divides fossil fishes, as he had previously done 

 their living representatives, into four gi-eat orders — the 

 Gaxoip, Pi.ACOiD, Ctenoid, and Cycloid — fomided on 

 the form of their scales. This arrangement would compre- 

 hend the immense variety of existing fishes, except two 

 genera, Mijxine and Petromyzon, which are totally destitute 

 of scales. 



Fossil 

 Ichthy- 

 ology. 



Fig. 1. 



Fig. 2. 



Fig. 3. 



I. — Ganoid fishes (figs. 1, 2, 3), (so designated from -yavo?, generally of a rhomboidal form, and imbricated, or applied 

 splendour) have shining scales, covered with a firm enamel, to each other as the slates on a roof. 



