ment provided at some time or other with epidermic appendages in the form of hairs. The young nourished for a shorter or longer time by means of a special fluid-the milk-secreted by special glands-the mammary glands. Animal viviparous.

As regards the general distribution in time of the Vertebrata, the earliest known traces of the existence of this sub-kingdom are found in the Upper Silurian Rocks. Here are the remains of Ganoid and Plagiostomous fishes; and we may fairly anticipate that further research will ultimately result in putting back the first appearance of Fishes at any rate to the Lower Silurian. The class of the Amphibians is not known to have come into existence prior to the commencement of the Carboniferous period, but it had attained a great development before the close of this epoch. The class of the true Reptiles is represented, by more or less doubtful examples only, in the newer Palæozoic deposits. In the Mesozoic Rocks, however, the development of this class was so great that the Secondary period has been termed the "Age of Reptiles." The class Aves is doubtfully represented by foot-prints in strata of the age of the Trias; but no Palæozoic remains of this class have been as yet detected. The earliest undoubted remains of Birds occur in the Jurassic series, and the class has continued to be represented more or less abundantly to the present day. Lastly, the class of the Mammalia, so far as at present known, finds its earliest fossil representative in strata of the age of the Trias (New Red Sandstone). The Mammals, however, cannot be said to be in any way abundant as fossils, till we reach the Eocene Tertiary. From this point onward the remains of Mammals are as abundant as, in the nature of the case, they could reasonably be expected to be.

CHAPTER XXVIII.

FISHES.

THE first class of the Vertebrata is that of the Fishes (Pisces). which may be broadly defined as including Vertebrate animals which are provided with gills throughout the whole of life; the heart, when present, consists (with one exception) of a single auric and a single ventricle; the blood is cold; the limbs, when present. are in the form of fins, or expansions of the integument.

In form, Fishes are adapted for rapid locomotion in water the shape of the body being such as to give rise to the least

possible friction in swimming. To this end also, as well as for purposes of defence, the body is usually enveloped with a coating of scales developed in the inferior or dermal layer of the skin. The more important modifications in the form of these dermal scales are as follows: I. Cycloid scales (fig. 272), consisting of thin, flexible, horny scales, circular or elliptical in shape, and having a more or less completely smooth outline. These are the scales which are characteristic of most of the ordinary bony fishes. II. Ctenoid scales (fig. 273), also consisting of thin horny plates, but having their posterior margins

Fig. 272.-Cycloid scale. Fig. 273.-Ctenoid scale.

Fig. 274.-Ganoid scale.

fringed with spines, or cut into comb-like projections. III. Ganoid scales, composed of an inferior layer consisting of bone, covered by a superficial layer of hard polished enamel (the socalled "ganoine"). These scales (fig. 274) are usually much larger and thicker than the ordinary scales, and though they are often articulated to one another by special processes, they only rarely overlap. IV. Placoid scales, consisting of detached bony grains, tubercles, or plates, of which the latter are not uncommonly armed with spines.

It is very important for the geologist to recognise the characters of these different scales, as he may have to decide upon the characters of a fossil fish merely from detached scales. Such decisions, however, are always more or less hazardous, since the scales of the different orders of the living fishes are not invariably of the same kind in all the forms of the order. Thus, ganoid scales are not peculiar to the order of the Ganoid fishes, but occur also in some of the Bony Fishes (Teleostei). The scales, also, form at best but one character, and they can hardly be said to constitute the most important character of any fish. A classification, therefore, which is based primarily upon the nature of the scales, necessarily is more or less "artificial," and is liable to bring into juxtaposition forms which have no real affinity to one another. For these reasons, most zoologists do not accept the classification of the Fishes into the four orders of the Cycloidei, Ctenoidei, Ganoidei, and Placoidei, since this classification, though sanctioned by such an eminent

authority as Professor Agassiz, is founded solely upon the nature of the integumentary covering. The paleontologist, however, whose materials often consist of nothing more than detached scales, is not rarely driven, by the necessity of the case, to provisionally classify his specimens in accordance with the nature of these appendages.

As regards their true osseous system or endoskeleton, Fishes vary very widely. In the Lancelet there can hardly be said to be any skeleton, the spinal cord being simply supported by the gelatinous notochord, which remains throughout life. In others the skeleton remains permanently cartilaginous; in others it is partially cartilaginous and partially ossified; and, lastly, in most modern fishes it is entirely ossified, or converted into bone. Taking a bony fish (fig. 275) as in this respect a typical example of the class, the following are the chief points in the osteology of a fish which require notice :

The vertebral column in a bony fish consists of vertebræ which are hollow at both ends, or biconcave, and are technically said to be "amphicœlous." The cup-like margins of the vertebral bodies are united by ligaments, and the cavities formed between contiguous vertebræ are filled with the gelatinous remains of the notochord. This elastic gelatinous substance acts as a kind of ball-and-socket joint between the bodies of the vertebræ, thus giving the whole spine the extreme mobility which is requisite for animals living in a watery medium. The ossification of the vertebræ is often much more imperfect than the above, but in no case except that of the Bony Pike (Lepidosteus) is ossification carried to a greater extent than this. In this fish, however, the vertebral column is composed of "opisthocœlous" vertebræ-that is, of vertebræ the bodies of which are concave behind and convex in front. The entire spinal column is divisible into not more than two distinct regions, an abdominal and a caudal region. The abdominal vertebræ possess a superior or neural arch (through which passes the spinal cord), a superior spinous process (neural spine), and two transverse processes to which the ribs are usually attached. The caudal vertebræ (fig. 275) have no marked transverse processes; but in addition to the neural arches and spines, they give off an inferior or hamal arch below the body of the vertebræ, and the hæmal arches carry inferior spinous processes (hæmal spines).

The ribs of a bony fish are attached to the transverse processes, or to the bodies, of the abdominal vertebræ, in the form of slender curved bones which articulate with no more than one vertebra each, and that only at a single point. Unlike the

ribs of the higher Vertebrates, the ribs do not enclose a thoracic cavity, but are simply imbedded in the muscles which bound the abdomen. Usually each rib gives off a spine-like bone, which is directed backwards amongst the muscles. Inferiorly the extremities of the ribs are free, or are rarely united to dermal ossifications in the middle line of the abdomen; but there is never any breast-bone or sternum properly so called.

Fig. 275.-Skeleton of the common Perch (Perca fluviatilis). Pectoral fin; One of the ventral fins; a Anal fin, supported upon interspinous bones (i); c Caudal fin; d First dorsal fin; d Second dorsal fin, both supported upon interspinous bones; ii Interspinous bones; Ribs; s Spinous processes of vertebra; h Hamal processes of vertebræ.

The only remaining bones connected with the skeleton of the trunk are the so-called interspinous bones (fig. 275, i ). These form a series of dagger-shaped bones plunged in the middle line of the body between the great lateral muscles which make up the greater part of the body of a fish. The internal ends or points of the interspinous bones are attached by ligament to the spinous processes of the vertebræ ; whilst to their outer ends are articulated the "rays" of the so-called "median" fins, which will be hereafter described. As a rule, there is only one interspinous bone to each spinous process, but in the Flat-fishes (Sole, Turbot, &c.) there are two.

Beside the fins which represent the limbs (pectoral and ventral fins), fishes possess other fins placed in the middle line of the body, and all of these alike are supported by bony spines or "rays," which are of two kinds, termed respectively "spinous rays" and "soft rays." The "spinous rays" are simple bony spines, apparently composed of a single piece each, but really consisting of two halves firmly united along the middle line. The "soft rays" are composed of several slender spines

proceeding from a common base, and all divided transversely into numerous short pieces. The soft rays occur in many fishes in different fins, but they are invariably found in the caudal fin or tail (fig. 275, c). The rays of the median fins, whatever their character may be, always articulate by a hinge-joint with the heads of the interspinous bones.

The skull of the bony fishes is an extremely complicated structure, and it is impossible to enter into its composition here. The only portions of the skull which require special mention are the bones which form the gill-cover or operculum. For reasons connected with the respiratory process in fishes, there generally exists between the head and the scapular arch a great cavity or gap on each side, within which are contained the branchia. The cavity thus formed opens externally on each side of the neck by a single vertical fissure or "gill-slit," closed by a broad flap, called the "gill-cover" or "operculum," and by a membrane termed the "branchiostegal membrane." The gill-cover (fig. 276, p, 0, s, i) is composed of a chain of broad flat bones, termed the opercular bones. Of these, the

Fig. 276.-Skull of Cod (Morrhua vulgaris)-Cuvier. a Urohyal; & Basihyal; Ceratohyal; d Branchiostegal rays; Præ-operculum; Operculum proper; s Sub-oper culum; i Inter-operculum; m Mandible; Intermaxillary bone.

innermost articulates with the skull (tympano-mandibular arch), and is called the "præ-operculum;" the next is a large bone