Fossils of the Upper Old Red Sandstone much more imperfectly preserved than those of the Lower.--The Causes obvious.--Difference between the two Groups, which first strikes the Observer, a Difference in Size.--The _Holoptychius_ a characteristic Ichthyolite of the Formation.--Description of its huge Scales.--Of its Occipital Bones, Fins, Teeth, and General Appearance.--Contemporaries of the _Holoptychius_.--Sponge-like Bodies.--Plates resembling those of the Sturgeon.--Teeth of various Forms, but all evidently the Teeth of Fishes.--Limestone Band, and its probable Origin.--Fossils of the Yellow Sandstone.--The _Pterichthys_ of Dura Den.--Member of a Family peculiarly characteristic of the System.--No intervening Formation between the Old Red Sandstone and the Coal Measures.--The _Holoptychius_ contemporary for a time with the _Megalichthys_.--The Columns of Tubal Cain. The different degrees of entireness in which the geologist finds his organic remains, depend much less on their age than on the nature of the rock in which they occur; and as the arenaceous matrices of the Upper and Middle Old Red Sandstones have been less favorable to the preservation of their peculiar fossils than the calcareous and aluminous matrices of the Lower, we frequently find the older organisms of the system fresh and unbroken, and the more modern existing as mere fragments. A fish thrown into a heap of salt would be found entire after the lapse of many years; a fish thrown into a heap of sand would disappear in a mass of putrefaction in a few weeks; and only the less destructible parts, such as the teeth, the harder bones, and perhaps a few of the scales, would survive. Now, limestone, if I may so speak, is the preserving salt of the geological world; and the conservative qualities of the shales and stratified clays of the Lower Old Red Sandstone are not much inferior to those of lime itself; while, in the Upper Old Red, we have merely beds of consolidated sand, and these, in most instances, rendered less conservative of organic remains than even the common sand of our shores, by a mixture of the red oxide of iron. The older fossils, therefore, like the mummies of Egypt, can be described well nigh as minutely as the existences of the present creation; the newer, like the comparatively modern remains of our churchyards, exist, except in a few rare cases, as mere fragments, and demand powers such as those of a Cuvier or an Agassiz to restore them to their original combinations. But cases, though few and rare, do occur in which, through some favorable accident connected with the death or sepulture of some individual existence of the period, its remains have been preserved almost entire; and one such specimen serves to throw light on whole heaps of the broken remains of its contemporaries. The single elephant, preserved in an iceberg beside the Arctic Ocean, illustrated the peculiarities of the numerous extinct family to which