The old red sandstone formation is very extensively distributed in the northern counties, forming a great belt round the coast from Caithness-shire to Aberdeenshire, and consisting of three well-marked divisions, the lower, middle, and upper series of beds. The strata flank the northern walls of the Grampians and their out-liers, traversing the great central or Caledonian valley for a hundred miles, and training round the western coast by Oban, the shores of Mull and Morven. They are of great thickness in many places; and in some of the beds, as at Cromarty, Lethen-bar, and Gamrie, contain nearly all the fossils peculiar to the formation. The order of Ganoid fishes, which afterward fulfill so distinguished a part in the kingdom of nature, is wholly absent from the silurian group, while, in the Devonian, nearly thirty genera, and considerably above sixty species, have been described and named. The scales of these creatures would appear to have been richly ornamented, enameled, and shining, and hence the term Ganoid applied to the order. In the northern districts, beyond Ben Mac-Dhui, the following genera, with several species belonging to each, have been found, namely, coccosteus, cheiracanthus, cheirolepis, dipterus, diplopterus, diplocanthus, glyptolepis, osteolepis, pterichthys. The principal localities of these fossils are—the Dipple on the Spey, Tynet Burn in Banffshire, Seat-Craig near Elgin, Altyre on the Findhorn, Clune, and Lethen-bar in Nairnshire, Gamrie, Cromarty, and various places in Sutherland and Caithness. Shetland is chiefly composed of the old red sandstone, which yields abundantly the fossils peculiar to the deposit. The formation extends through the Orkney islands, inexhaustibly fertile in organic remains, and among which have been found plates and fragments of the Asterolepis, the largest of all the genera belonging to the period: the head and jaws, at least, appear to have been of enormous dimensions, and portions of the inner skeleton must have been bony, contrary to the general cartilaginous structure of the class. The Placoids of the subjacent rocks have many resemblances to the cestracions, centrinæ, and spinaxes of our present seas, their scales being set like plates at irregular distances over the body. The Ganoids, on the other hand, whose scales were continuous, and enveloped the entire animal, have no affinities to any living types.