The _heterocercal_ fish, it will be seen, are unequally lobed, that is, the spinal vertebræ are prolonged into the upper lobe of the tail, as seen in the shark, and of which our own dog-fish is an example; while the homocercal fish are equally lobed, and the spine does not extend into either. The fossil fish of the old red sandstone belong almost, if not entirely, to the classes of fish that have ganoid or placoid scales, and heterocercal tails; and of these fish we will now say a few words of the four most remarkable specimens of the one thousand and upwards fossil species that have been discovered, and which can only be known familiarly by accomplished geologists in the ichthyolite department. [Illustration: Fossil] 1. Here is a drawing of the Cephalaspis,[48] or buckler-headed fish. What an extraordinary looking creature this is! Like the crescent shape of a saddler’s knife without the handle—broad and flat, with points on each side running down, ever fixed in warlike attitude against its enemies—it reminds one of an extinct trilobite, and of a living sole or ray, at the same time; and one can easily fancy how hard it must have been for its ancient foes to swallow down so singular and so knife-like looking a creature. This is one of the curious organisms of old life discovered in Cromarty, Herefordshire, and in Russia, the original of which, restored in the drawing, seldom if ever exceeded seven inches. Let us look now at another curiosity from the same quarter. [Illustration: Fossil] 2. Here is a drawing of the Coccosteus,[49] or berry-boned fish. This creature is equally singular with his long extinct neighbour. Hugh Miller’s description is the best, and as he was its discoverer, let us give it. “The figure of the Coccosteus I would compare to a boy’s kite; there is a rounded head, a triangular body, a long tail attached to the apex of the triangle, and arms thin and rounded where they attach to the body, and spreading out towards their termination, like the ancient one-sided shovel which we see sculptured on old tombstones, or the rudder of an ancient galley. A ring of plates, like the ring-stones of an arch, runs along what we may call the hoop of the kite. The form of the key-stone plate is perfect; the shapes of the others are elegantly varied, as if for ornament; and what would be otherwise the opening of the arch is filled up with one large plate of an outline singularly elegant.”[50] [Illustration: Fossil]