Agassiz, in the course of his late visit to Scotland, found six species of the Pterichthys[J]—three of these, and the wings of a fourth, in the collection of the writer. The differences by which they are distinguished may be marked by even an unpractised eye, especially in the form of the bodies and wings. Some are of a fuller, some of a more elongated, form; in some the body resembles a heraldic shield, of nearly the ordinary shape and proportions; in others the shield stretches into a form not very unlike that of a Norway skiff, from the midships forward. In some of the varieties, too, the wings are long and comparatively slender; in others shorter, and of greater breadth: in some there is an inflection resembling the bend of an elbow; in others there is a continuous swelling from the termination to the shoulder, where a sudden narrowing takes place immediately over the articulation. I had inferred somewhat too hurriedly, though perhaps naturally enough, that these wings, or arms, with their strong sharp points and oar-like blades, had been at once paddles and spears—instruments of motion and weapons of defence; and hence the mistake of connecting the creature with the Chelonia. I am informed by Agassiz, however, that they were weapons of defence only, which, like the occipital spines of the river bull-head, were erected in moments of danger or alarm, and at other times lay close by the creature's side; and that the sole instrument of motion was the tail, which, when covered by its coat of scales, was proportionally of a somewhat larger size than the tail shown in the print, which, as in the specimens from whence it was taken, exhibits but the obscure and uncertain lineaments of the skeleton. The river bull-head, when attacked by an enemy, or immediately as it feels the hook in its jaws, erects its two spines at nearly right angles with the plates of the head, as if to render itself as difficult of being swallowed as possible. The attitude is one of danger and alarm; and it is a curious fact, to which I shall afterwards have occasion to advert, that in this attitude nine tenths of the Pterichthys of the Lower Old Red Sandstone are to be found. We read in the stone a singularly preserved story of the strong instinctive love of life, and of the mingled fear and anger implanted for its preservation—"The champions in distorted postures threat." It presents us, too, with a wonderful record of violent death falling at once, not on a few individuals, but on whole tribes.
[J] Agassiz now reckons ten distinct species of Pterichthys—P. arenatus, P. cancriformis, P. cornutus, P. major, P. Milleri, P. latus, P. oblongus, P. productus, P. testudinarius, and P. hydrophilus; of these, nine species belong to the Lower, and one—the Pterichthys hydrophilus—to the Upper Old Red Sandstone.
PLATE III.
Next to the Pterichthys of the Lower Old Red I shall place its contemporary the Coccosteus of Agassiz, a fish which, in some respects, must have somewhat resembled it. Both were covered with an armor of thickly tubercled bony plates, and both furnished with a vertebrated tail. The plates of the one, when found lying detached in the rock, can scarcely be distinguished from those of the other: there are the same marks, as in the plates of the tortoise, of accessions of growth at the edges—the same cancellated bony structure within, the same kind of tubercles without. The forms of the creatures themselves, however, were essentially different. I have compared the figure of the Pterichthys, as shown in some of my better specimens, to that of a man with the head cut off at the shoulders, one of the legs also wanting, and the arms spread to the full. The figure of the Coccosteus I would compare to a boy's kite. (See [Plate III.], fig. 1.) 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.[K] The manner in which the plates are arranged on the head is peculiarly beautiful; but I am afraid I cannot adequately describe them. A ring of plates, like the ring-stones of an arch, runs along what may be called the hoop of the kite; the form of the keystone-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. A single plate, still larger than any of the others, covers the greater part of the creature's triangular body, to the shape of which it nearly conforms. It rises saddle-wise towards the centre: on the ridge there is a longitudinal groove ending in a perforation, a little over the apex, ([Plate III.], fig. 2;) two small lateral plates on either side fill up the base of the angle; and the long tail, with its numerous vertebral joints, terminates the figure.
[K] I have since ascertained that these seeming arms or paddles were simply plates of a peculiar form.
Does the reader possess a copy of Lyell's lately published elementary work, edition 1838? If so, let him first turn up the description of the Upper Silurian rocks, from Murchison, which occurs in page 459, and mark the form of the trilobite Asaphus caudatus, a fossil of the Wenlock formation. (See Sil. Sys., [Plate VII.]) The upper part, or head, forms a crescent; the body rises out of the concave with a sweep somewhat resembling that of a Gothic arch; the outline of the whole approximates to that of an egg, the smaller end terminating in a sharp point. Let him remark, further, that this creature was a crustaceous animal, of the crab or lobster class, and then turn up the brief description of the Old Red Sandstone in the same volume, page 454, and mark the form of the Cephalaspis, or buckler-head—a fish of a formation immediately over that in which the remains of the trilobite most abound. He will find that the fish and the crustacean are wonderfully alike. The fish is more elongated, but both possess the crescent-shaped head, and both the angular and apparently jointed body.[L] They illustrate admirably how two distinct orders may meet. They exhibit the points, if I may so speak, at which the plated fish is linked to the shelled crustacean. Now, the Coccosteus is a stage further on; it is more unequivocally a fish. It is a Cephalaspis with an articulated tail attached to the angular body, and the horns of the crescent-shaped head cut off.
[L] Really jointed in the case of the trilobite; only apparently so in that of the Cephalaspis. The body of the trilobite, like that of the lobster, was barred by transverse, oblong, overlapping plates, and between every two plates there was a joint; the body of the Cephalaspis, in like manner, was barred by transverse, oblong, overlapping scales, between which there existed no such joints. It is interesting to observe how nature, in thus bringing two such different classes as fishes and crustacea together, gives to the higher animal a sort of pictorial resemblance to the lower, in parts where the construction could not be identical without interfering with the grand distinctions of the classes.
Some of the specimens which exhibit this creature are exceedingly curious. In one, a coprolite still rests in the abdomen; and a common botanist's microscope shows it thickly speckled over with minute scales, the indigestible exuviæ of fish on which the animal had preyed. In the abdomen of another we find a few minute pebbles—just as pebbles are occasionally found in the stomach of the cod—which had been swallowed by the creature attached to its food. Is there nothing wonderful in the fact, that men should be learning at this time of day how the fishes of the Old Red Sandstone lived, and that there were some of them rapacious enough not to be over nice in their eating?