The recent bony fishes are divided, according to the Cuvierian system of classification, into two great orders, the soft-finned and the thorny-finned order--the _Malacopterygii_ and the _Acanthopterygii_. In the former the rays of the fins are thin, flexible, articulated, branched: each ray somewhat resembles a jointed bamboo; with this difference, however, that what seems a single ray at bottom, branches out into three or four rays a-top. In the latter, (the thorny-finned order,)--especially in their anterior dorsal, and perhaps anal fins,--the rays are stiff continuous spikes of bone, and each stands detached as a spear, without joint or branch. The perch may be instanced as a familiar illustration of this order--the gold-fish of the other. Now, between the fins of two sets--shall I venture to say orders?--of the ichthyolites of the Lower Old Red Sandstone, an equally striking difference obtains. The fin of the _Osteolepis_, with its surface of enamelled and minutely jointed bones, I have already described as a sort of bird-wing fin. The naked rays, with their flattened surfaces, lay thick together as feathers in the wing of a bird--so thick as to conceal the connecting membrane; and fins of similar construction characterized the families of the _Dipterus_, _Diplopterus_, _Glyptolepis_, _Cheirolepis_, _Holoptychius_, and, I doubt not, many other families of the same period, which await the researches of future discoverers. But the fins of another set of ichthyolites, their contemporaries, may be described as bat-wing fins: they presented to the water a broad expanse of membrane; and the solitary ray which survives in each was not a jointed, but a continuous spear-like ray. The fins of this set, or order, are thorny-fins, like those of the _Acanthopterygii_; the anterior edge of each, with the exception of, perhaps, the caudal fin, which differs in construction from the others, is composed of a strong, bony spike. Such, with some tacit reference, perhaps, to the similar Cuvierian principle of classification, were the distinctive differences, on the strength of which I used to arrange two of my groups of fossils on the granitic boulder; and the influence of the same principle, almost instinctively exerted,--for, in writing the previous pages, I scarce thought of its existence,--has, I find, given to each group its own chapter.