CHAPTER V.

The Classifying Principle, and its Uses.-Three Groupes of Ichthyolites among the Organisms of the Lower Old Red Sandstone.-Peculiarities of the Third Group.-Its Varieties.-Description of the Cheiracanthus. Of two unnamed Fossils of the same Order.-Microscopic Beauty of these Ancient Fish. Various Styles of Ornament which obtain among them.-The Molluscs of the Formation.-Remarkable chiefly for the Union of Modern with Ancient Forms which they exhibit. Its Vegetables.-Importance and Interest of the Record which it furnishes.

THERE rests in the neighbourhood of Cromarty, on the upper stratum of one of the richest ichthyolite beds I have yet seen, a huge water-rolled boulder of granitic gneiss, which must have been a traveller in some of the later periods of geological change, from a mountain range in the interior highlands of Rossshire, more than sixty miles away. It is an uncouthlooking mass, several tons in weight, with a flat upper surface, like that of a table; and as a table, when engaged in collecting my specimens, I have often found occasion to employ it. I have covered it over, times without number, with fragments of fossil fish,-with plates, and scales, and jaws, and fins, and, when the search proved successful, with entire ichthyolites. Why did I always arrange them, almost without

thinking of the matter, into three groupes? Why, even when the mind was otherwise employed, did the fragments of the Coccosteus and Pterichthys come to occupy one corner of the stone, and those of the various fish just described another corner, and the equally well-marked remains of a yet different division a third corner? The process seemed almost mechanical, so little did it employ the attention, and so invariable were the results. The fossils of the surrounding bed always found their places on the huge stone in three groupes, and at times there was yet a fourth group added,—a group whose organisms belonged not to the animal, but the vegetable kingdom. What led to the arrangement? or in what did it originate? In a principle inherent in the human mind,—that principle of classification which we find pervading all science,which gives to each of the many cells of recollection its appropriate facts,-and without which all knowledge would exist as a disorderly and shapeless mass, too huge for the memory to grasp, and too heterogeneous for the understanding to employ. I have described but two of the groupes, and must now say a very little about the principle on which, justly or otherwise, I used to separate the third, and on the distinctive differences which rendered the separation so easy.

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 atop. 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 disBut the fins of another set of ichthyolites, their cotemporaries, may be described as batwing 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 classifi

coverers.

cation, were the distinctive differences, on the strength of which I used to arrange two of my groupes 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.

Of the membranous-finned and thorny-rayed order of ichthyolites, the Cheiracanthus, or thorny-hand, i. e. pectoral, may be regarded as an adequate representative (see Plate VII. fig. 1). The Cheiracanthus must have been an eminently handsome little fish,-slim, tapering, and described in all its outlines, whether of the body or the fins, by gracefully-waved lines. It is, however, a rare matter to find it presenting its original profile in the stone ;-none of the other ichthyolites are so frequently distorted as the Cheiracanthus. It seems to have been more a cartilaginous and less an osseous fish than most of its cotemporaries. However perfect the specimen, no part of the internal skeleton is ever found, not even when scales as minute as the point of a pin are preserved, and every spine stands up in its original place. And hence, perhaps, a greater degree of flexibility, and consequent distortion. The body was covered with small angular scales, brightly enamelled, and delicately fretted into parallel ridges that run longitudinally along the upper half of the scale, and leave the posterior portion of it a smooth glittering surface (see Plate VII. fig. 2). They diminish in size towards the head, which, from the faint stain left on the stone, seems to have been composed of cartilage exclusively, and either covered with skin

or with scales of extreme minuteness. The lower edge of the operculum bears a tagged fringe, like that of a curtain. The tail, a fin of considerable power, had the unequal-sided character common to the formation; and the slender and numerous rays on both sides are separated by so many articulations as to present the appearance of parallelogrammical scales. The other fins are comparatively of small size. There is a single dorsal placed about two-thirds the entire length of the creature adown the back; and exactly opposite its posterior edge is the anterior edge of the anal fin. The ventral fins are placed high upon the belly, somewhat like those of the perch; the pectorals only a little higher. But it is rather in the construction of the fins than their position that the peculiarities of the Cheiracanthus are most marked. The anterior edge of each, as in the pectorals of the existing genera Cestracion and Chimara, is formed of a strong large spine. In the Chimera Borealis, a cartilaginous fish of the Northern Ocean, the spine seems placed in front of the weaker rays, just, if I may be allowed the comparison, as, in a line of mountaineers engaged in crossing a swollen torrent, the strongest man in the party is placed on the upper side of the line, to break off the force of the current from the rest. In the Cheiracanthus, however, each fin seems to consist of but a single spine, with an angular membrane fixed to it by one of its sides, and attached to the creature's body on the other. Its fins are masts and sails, the spine representing the mast, and the membrane the sail; and it is a curious characteristic of the order, that the membrane, like the body, of the ichthyolite,