The geological history of fishes presents some points of peculiar interest. Of all the classes of the great sub-kingdom Vertebrata, the fishes are the lowest in point of organisation. It might therefore have been reasonably expected that they would present us with the first indications of vertebrate life upon the globe; and such is indeed the case. After passing through the enormous group of deposits known as the Lauren-tian, Huronian, Cambrian, and Lower Silurian formations - representing an immense lapse of time, during which, so far as we yet know with certainty, and leaving the "Conodonts " out of sight, no vertebrate animal had been created - we find in the Upper Silurian rocks the first traces of fish. The earliest of these, in Britain, are found in the base of the Ludlow rocks (Lower Ludlow Shale), and belong to the Placoganoid genus Pteraspis. Also in the Ludlow rocks, but at the summit of their upper division, are found fin-spines and shagreen, probably belonging to Cestraciont fishes - that is to say, to fishes of as high a grade of organisation as the Elasmobranchii. So abundant are the remains of fishes in the next great geological epoch - namely, the Devonian or Old Red Sandstone - that this period has frequently been designated the "Age of Fishes." Most of the fishes of the Old Red Sandstone belong to the order Ganoidei, but the order Dipnoi appears to be also represented. In the Carboniferous and Permian rocks which close the Palaeozoic period, most of the fishes are still Ganoid, but the former contain the remains of many Elasmobranchii. At the close of the Palaeozoic and the commencement of the Mesozoic epoch, the Ganoid fishes begin to lose that predominant position which they before occupied, though they continue to be represented through the whole of the Mesozoic and Kainozoic periods up to the present day. The Ganoids, therefore, are an instance of a family which has endured through the greater part of geological time, but which early attained its maximum, and has been slowly dying out ever since. Towards the close of the Mesozoic period (in the Cretaceous period) the great family of the Teleostean or Bony Fishes is for the first time known certainly to have made its appearance. The families of the Marsipobranchii and Pharyn-gobranchii have not left, so far as is known, any traces of their existence in past time. Judging from analogy, however, it is highly probable that both of these must have had a vast antiquity, and it is not impossible that the so-called "Cono-donts" of the Palaeozoic period may yet be shown to be the teeth of fishes allied to the Lampreys.
Leaving these unrepresented orders out of consideration, the following are the chief facts as to the geological distribution of the other great groups: