PLACOID, CTENOID, AND CYCLOID FISHES. 71 Paleozoic strata; they become more numerous in the Carboniferous series; they are very numerous in the Lias and Chalk formations; but there they cease almost entirely,—the strata of the Tertiary series scarcely containing any of them, and the Port-Jackson Shark being the only representative of this family at the present day.-Intermediate between these and the ordinary Sharks was another family, to which the name of Hybodonts has been given. The teeth of this division were stronger and blunter than those of the true Sharks, but were not so much flattened as those of the Cestracionts; and they seem to have been adapted for cutting, tearing, and bruising substances of considerable hardness. The Fishes of this family seem to have made their first appearance in the later part of the Coal formations; they were very abundant during the Oolitic period; but ceased entirely at the commencement of the Chalk deposit.-Lastly, the ordinary Sharks, constituting the Squaloid family, which are distinguished by their sharp lancet-like teeth, have no representatives among the Fossil Fishes of older date; but their remains are first found in the Chalk formations, and extend through all the newer strata down to the present time.-The Rays, also, of the earlier periods had teeth more flattened than those of later epochs; and we find from their fossil remains, that some of the forms, which are now regarded as exceptional or aberrant, were formerly more abundant. Thus of the Myliobatis or Eagle-Ray (§ 584), of which five species are at present known, fifteen fossil species have been discovered. 590. The Ctenoid and Cycloid Fishes make their first appearance in the Chalk formation; when all the previouslyexisting genera of the Ganoid and Placoid orders had become extinct, and when the new ones that were brought into existence were (as we have seen) far less numerous than before. There is, then, a sort of boundary line at the base of the Cretaceous or Chalk deposits, which divides the class of Fishes in a most remarkable manner;-all those below that line, in the order of the strata, or (in other words) all those which existed at a period anterior to the deposition of the Chalk, having belonged to the two first orders, those with enamelled scales;—whilst by far the 72 GENERAL RELATIONS OF FOSSIL FISHES. larger proportion of those existing at a subsequent time, as at the present epoch, belong to the two orders with horny scales, which comprehend at least three-fourths of the 8000 living species of Fishes known to Naturalists. Of the fossils of these last orders, which abound in the formations of the Tertiary period, it is sufficient to say, that they may nearly all be referred to families which have been described as now existing; that many of them belong to the same genera with recent Fishes; but that it is doubtful if any of them are of the same species with those now tenanting our seas. 591. The sketch here given of the principal groups of Fossil Fishes, is sufficient to illustrate some very interesting points in the history of this class. In the first place, we see that the firstcreated Vertebrated animals so far resembled the Invertebrated classes, as to possess a very dense external skeleton ; whilst their internal skeleton was so soft, as not to be capable of being preserved; and it appears to have Leen to the Crustacea, which are among the highest of the Articulated series, that these Fishes were most nearly related. On the other hand, among the Fishes of the present time, the Cyclostomata, which present the characters of the Vertebrata in their most imperfect form (§ 585), are rather analogous to the Annelida, or Worm tribe; with which, indeed, some of them were actually classed by Linnæus.—We may next observe, that the covering of dense enamelled scales, in which all the Fishes of the earlier formations were inclosed, rendered necessary a peculiar conformation in the mouths of those, which were destined to prey upon them and to restrain their multiplication. Thus we find the mouths of the predaceous Ganoid fishes, such as the Holoptychius and its allies, and in those of the Cestracionts and Hybodonts, a pavement-like covering of flat enamelled teeth, adapted to crush the hardest substances. But when the enamelled-scaled Fishes had given place to those furnished with a softer covering, we find these enamelled pavements of the jaws replaced by sharp cutting teeth, the predaceous Ganoids disappearing altogether, and the Cestracionts and Hybodonts giving place to the Sharks.-What was the purpose for which all the earlier races of Fishes were provided GENERAL RELATIONS OF FOSSIL FISHES. 73 with the enamelled covering so often alluded to, and why these races should have given place at a later period to others so differently constituted, we have no certain means of knowing. It has been suggested by Dr. Buckland, that the purpose of this cuirass may have been, to defend the bodies of the animals against waters that were warmer, or subject to more sudden changes of temperature than could be endured by animals, whose skin was protected only by such thin and often disconnected coverings, as the miembranous and horny scales of most modern Fishes. Such changes of temperature were more frequent, there is good reason to believe, in the earlier epochs of the Earth's history, than they are at the present time, for reasons which will be elsewhere explained. (See Treatise on GEOLOGY.) 592. It may be further remarked, that the history of the Fossil Fishes most clearly proves, that the first-created forms of Animal life were not the least perfect, as some have maintained; and that there is not a regular succession of new races, increasing in elaborateness of structure, from the oldest to the most recent formations. For among the earliest races of Fishes, we find that those of the Sauroid family, which had many characters of elevation, held a conspicuous place; and that, when these were replaced by the class of Reptiles, which was called into existence as soon as there was any land for its habitation, they ceased to exist, and were succeeded by races of Fishes which must be regarded as lower in the scale. CHAPTER VII. GENERAL CHARACTERS OF ARTICULATED ANIMALS. 593. THE animals of which is composed the Sub-Kingdom ARTICULATA, not only present an internal structure which is essentially different from that of the other three divisions of the Animal series, but are likewise distinguished by external characters, which are usually so definite and evident, that they may be almost always recognised at the first glance. Their entire body, in fact, is divided (more or less obviously) into segments; and seems to be composed of a series of rings placed in a line. In some Articulated animals, this annular (ring-like) appearance results merely from a certain number of transverse folds, which furrow the skin, and encircle the body; but in the greater number, the animal is enclosed in a sort of solid armour, composed of a series of rings, united one to another in such a manner as to allow them a certain degree of movement. This envelope has uses analogous to that of the internal framework of Vertebrated animals; for it determines the general form of the body; it protects the soft parts; it gives points of attachment to the muscles; and furnishes them with levers, by their action on which the movements may be effected with precision and rapidity. Hence it may be termed an external skeleton. It must be borne in mind, however, that the hard substance of which it is composed has no analogy in structure, or in GENERAL STRUCTURE OF ARTICULATED ANIMALS. 75 mode of growth, with the bones of Vertebrated animals. It consists, in fact, of the skin, consolidated by the deposition of horny or calcareous matter in its substance; and instead of being able to grow in all directions in the manner of bone, it undergoes very little change when it is once fully formed, and it cannot be made to increase in size except by addition to its edges. 594. The different rings, or segments, of the body of an Articulated animal, always bear a strong resemblance to each other; and sometimes, as in the Iulus (Fig. 287), and the Scolopendra, or Centipede, they seem like actual repetitions of each other. FIG. 288.-VERTICAL SECTION OF A SEGMENT OF AN ANNELIDE, BELONGING Each ring may bear two pairs of appendages, or members; the one belonging to its dorsal arch, or the upper part of the segment; whilst the other belongs to the ventral arch, or the under part of the ring. In those Articulata in which the principle of "division of labour" does not yet manifest itself,—that is, in whose bodies we find a number of similar parts adapted to perform the same functions, instead of an assemblage of different parts, constructed each for its own special purpose,—all the segments are provided with these appendages, and their number is sometimes extremely great. But in all the higher tribes, we find the appendages of certain segments attaining a high de,gree of development; whilst, by a sort of compensating principle, the others present themselves as mere rudiments, or are not even at all discoverable. In general the appendages of the ventral arch are alone developed; and the variety of forms which they |