It is to the two last of these groups that the Devonian fishes belong, and they are more specially referable to the _Ganoids_. The order of the Ganoid fishes at the present day comprises but some seven or eight genera, the species of which principally or exclusively inhabit fresh waters, and all of which are confined to the northern hemisphere. As compared, therefore, with the Bony fishes, which constitute the great majority of existing forms, the Ganoids form but an extremely small and limited group. It was far otherwise, however, in Devonian times. At this period, the bony fishes are not known to have come into existence at all, and the Ganoids held almost undisputed possession of the waters. To what extent the Devonian Ganoids were confined to fresh waters remains yet to be proved; and that many of them lived in the sea is certain. It was formerly supposed that the Old Red Sandstone of Scotland and Ireland, with its abundant fish-remains, might perhaps be a fresh-water deposit, since the habitat of its fishes is uncertain, and it contains no indubitable marine fossils. It has been now shown, however, that the marine Devonian strata of Devonshire and the continent of Europe contain some of the most characteristic of the Old Red Sandstone fishes of Scotland; whilst the undoubted marine deposit of the Corniferous limestone of North America contains numerous shark-like and Ganoid fishes, including such a characteristic Old Red genus as _Coccosleus_. There can be little doubt, therefore, but that the majority of the Devonian fishes were truly marine in their habits, though it is probable that many of them lived in shallow water, in the immediate neighbourhood of the shore, or in estuaries. [Illustration: Fig. 102.--Fishes of the Devonian rocks of America. a, Diagram of the jaws and teeth of _Dinichthys Hertzeri_, viewed from the front, and greatly reduced; b, Diagram of the skull of _Macropetalichthys Sullivanti_, reduced in size; c, A portion of the enamelled surface of the skull of the same, magnified; d, One of the scales of _Onychodus sigmoides_, of the natural size; e, One of the front teeth of the lower jaw of the same, of the natural size: f, Fin-spine of _Machoeracanthus major_, a shark-like fish, reduced in size. (After Newberry.)] [Illustration: Fig. 103.--_Cephalaspis Lyellii_. Old Red Sandstone, Scotland. (After Page.)] [Illustration: Fig. 104.--_Pterichthys cornutus_. Old Red Sandstone, Scotland. (After Agassiz.)]