ON THE FOSSIL FISH OF THE DEVONIAN SYSTEM. 85 



is it here the place to examine, under this point of view, the invertebrated ani- 

 mals, the study of which has likewise red me to very curious considerations ; 

 I will confine myself solely to the mentioning, in relation to the fishes of the 

 old red sandstone, that the small size of the great majority of the species, 

 compared with those of more recent periods, is a fact agreeing with what is 

 observed relatively to the first development of the greater part of the classes 

 of the animal kingdom. The genera of fish of the old red sandstone, the 

 species of which are of middle or small size, are the following : — Pterichthys, 

 Cephalaspis, Osteolepis, Dipterus, Glyptolepis, Acanthodes, Diplacanthus, 

 Cheir acanthus, and Cheirolepis ; and I mean by species of middle or of small 

 size in the class of fish, such as do not exceed the length of one or two feet. 

 The genera Diplopterus, Coccosteus, and the four Placoidian genera of this 

 formation, contained species certainly larger, but which nevertheless appear 

 to me not to have exceeded two or three feet in length. The genera Holo* 

 ptychius,Dendrodus, and Platygnathus, alone contained species of larger size ; 

 but I doubt if any one of them has attained the dimensions of our Sharks 

 and our Sword-fish, and much less those of the large Tunny. 



The close relation which appears to exist between the size of animals and 

 their organization, is evident in nearly all the families of the animal kingdom; 

 to be convinced of this, it suffices to pass in review a small collection only of 

 animals. 



Nothing is more striking in a museum arranged systematically, than the 

 conformity in size of species of the same family, where the extremes of the 

 differences are generally circumscribed within very narrow limits. How little 

 do the Quadrumana differ in reality one from the other in size ; what uni- 

 formity in this respect in their ensemble, among the Cheiroptera, the Insecti- 

 vora, the Rodents, the Ruminants; among the birds of prey, the Granivora, 

 the Chelonians, the Anourous Batrachians ; among Insects, taken as a whole, 

 compared with . other classes ; among Infusoria, &c. &c. The same rela- 

 tions equally exist between the families of Fishes ; and the diversity in size 

 which I have just noticed, among the species of different genera of this 

 formation, is indeed the first indication of the diversity of the types to 

 which they belong. I will point out, in the first place, four genera of the 

 Placoidian order, the genera Ctenacanthus, Onchus, Ctenoptychius and Pty- 

 chacanthus, which are provided with spinous rays to the dorsal fins, re- 

 sembling the gigantic Ichthyodorulites of the carboniferous and Jurassic for- 

 mations, but differing in their less considerable size; they are distinguished 

 among themselves by the forms and the ornaments of their rays. In the order 

 of Ganoid fishes, the genera Acanthodes, Diplacanthus, Cheir acanthus, and 

 Cheirolepis, present themselves at first sight as a separate group ; for although 

 covered, like the others, with enamelled scales, these are so small that they im- 

 part to the skin the appearance of shagreen. The manner in which the fins 

 are sustained by spinous rays, or the absence of these rays, and the position 

 of the fins themselves, have served as characters in the establishing of these 

 genera. The genera Pterichthys, Coccosteus, and Cephalaspis, form a second 

 group exceedingly curious : the considerable development of the head, its 

 size, the large .plates which cover it, and which likewise extend over the 

 greater portion of the trunk, and the moveable appendages in the form of a 

 wing, placed on the side of the head, give to them the most remarkable ap- 

 pearance. It is these peculiarities, indeed, which caused the class to which 

 these genera belong for a long time to be misunderstood. The large bony and 

 granulated plates of Coccosteus led to their being considered as belonging to 

 Trionyx ; and it will be a sufficient excuse for this error to call to recollection 

 that the greatest anatomist of our age had sanctioned this approximation. 



