In closing our review of the old red sandstone, we shall briefly state the principles of classification of fossil fishes, as determined by M. Agassiz, from which it will be seen by the earliest types of the marine vertebrata, while admirably suited to the perturbed condition of the element in which the strata were formed, differ widely in their structure from all existing races. The fishes of the present era, it is well known, are divided into two great classes, the cartilaginous and the osseous. In the former are comprehended the sharks, rays, and sturgeons of our present seas; the latter embrace the salmon, cod, herring, and the various kinds possessed of similar forms. The bony structure in all the cartilaginous class is soft, destitute of fibers, and contains scarcely a trace of earthy or calcareous matter. The osseous fishes, on the other hand, are constructed internally of true bone, composed of calcareous matter, like that of birds and quadrupeds, which is possessed of a fibrous arrangement, of great hardness and capable of long endurance. Now, it would appear that the fishes of the old red sandstone belong almost exclusively to the cartilaginous class. The internal frame was composed chiefly of this soft, soluble substance; hence it is that no portion of the inner body of the fish, in any of the fossil specimens, remains.—The teeth and scales, with fragments of the bones of the head, are all that have survived, but so hard and enduring has been the scaly outer coating, that the figure and contour of the animal have been preserved entire. The specimen of Holoptychius Andersoni, from Dura Den, is still enveloped in its original covering, not a scale in the whole body displaced or missing, the head and belly slightly compressed, while the posterior ridge of the back and tail is sharp and angular. Here, then, in this class of animal life, we find that what is defective in the internal structure—if it be a defect—is completely supplied in the outer appendages, whereby the fishes which have the softest bodies are possessed of hard, horny skins, coated with enamel. Their bones are thus all on the surface, sometimes in the form of scales; sometimes assuming the shape of spines and tubercles; now in small, now in large plates; and often disposed in the most singular and grotesque arrangements, as in the genus coccosteus, or the osteolepis, whose entire skull consisted of shining naked bone, and in the cheiracanthus, a creature possessed of fins scaled and enameled all over.