called the "operculum" proper; and the remaining two bones, called respectively the "sub-operculum" and "inter-operculum," form, with the operculum proper, the edge of the gillcover. These various bones are united together by membrane, and they form collectively a kind of movable door, by means of which the branchial chamber can be alternately opened and shut. Besides the gill-cover, however, the branchial chamber is closed by a membrane called the "branchiostegal membrane," which is attached to the os hyoides. The membrane is supported and spread out by a number of slender curved spines, which are attached to the lateral branches of the hyoid bone, act very much as the ribs of an umbrella, and known as the "branchiostegal rays" (fig. 276, d).

The limbs of fishes depart considerably from the typical form exhibited in the higher Vertebrates. One or both pairs of limbs may be wanting, but when present the limbs are always in the form of fins-that is, of expansions of the integument strengthened by bony or cartilaginous fin-rays. The anterior limbs are known as the pectoral fins, and the posterior as the ventral fins; and they are at once distinguished from the so-called "median" fins by being always disposed in pairs, usually symmetrically. Hence they are often spoken of as the paired fins.

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The fore-limbs or pectoral fins possess in a modified form most of the bones which are present in the anterior extremities of the higher Vertebrata. They vary much in size and in other characters. Sometimes they are enormously expanded, as in the Flying-fish (Exocatus); and at other times they form merely a pair of paddles, as in the extinct Pterichthys. The hindlimbs or ventral fins are wanting in many fishes, and they are less developed and less fixed in position than are the pectorals. In some cases the ventral fins are abdominal" in position, and are placed more or less towards the hinder part of the body (as in the Sharks, Ganoids, and Mud-fishes). In other cases, they are thoracic," that is, they are placed beneath the pectorals; and in some cases they are situated on the sides of the neck in advance of the pectorals, when they are said to be "jugular." In these cases, the pelvic arch is attached to the pectoral arch, and is therefore wholly removed from its normal position.

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In addition to the pectoral and ventral fins-the homologues of the limbs-which may be wanting, fishes are furnished with certain other expansions of the integument, which are "median" in position, and must on no account be confounded with the true "paired" fins. These median fins are variable

in number, and in some cases there is but a single fringe running round the posterior extremity of the body. In all cases, however, the median fins are azygous"—that is to say, they occupy the middle line of the body, and are not symmetrically disposed in pairs. Most commonly, the median fins consist of one or two expansions of the dorsal integument, called the "dorsal fins" (fig. 277, d d'); one or two on the ventral surface near the anus-the "anal fins" (fig. 277, a); and a broad fin at the extremity of the vertebral column, called the "caudal fin" or tail (c). In all cases, the rays which support the median fins are articulated with the so-called interspinous bones, which have been previously described. Though called "median," from their position in the middle line of the body, and from their being unpaired, the median fins of Fishes, as shown by Goodsir and Humphrey, are truly to be regarded as formed by the coalescence of two lateral elements in the mesial plane of the body.

V..

Fig. 277.-Outline of a fish (Perca granulata), showing the paired and unpaired fins. One of the pectoral fins: One of the ventral fins; d First dorsal fin; d' Second dor

sal fin; a Anal fin; c Caudal fin.

The caudal fin or tail of fishes is always set vertically at the extremity of the spine, so as to work from side to side, and it is the chief organ of progression in the fishes. In its vertical position and in the possession of fin-rays, it differs altogether from the horizontal integumentary expansion which constitutes the tail of the Whales, Dolphins, and Sirenia (Dugong and Manatee). In the form of the tail, fishes exhibit two very distinct types of structure, termed respectively the "homocercal" and "heterocercal" type of tail (fig. 278). The homocercal tail is the one which most commonly occurs in our modern fishes, and it is characterised by the fact that the two lobes of

the tail are equal, and the vertebral column, instead of being prolonged into the upper lobe of the tail, stops short at its base. In the heterocercal tail, on the other hand, the vertebral column is prolonged into the up

per lobe of the tail, so that the tail becomes unequally lobed, its greater portion being placed below the spine. Even where the vertebral column is not prolonged into the upper lobe, the tail may nevertheless become heterocercal, in consequence of a great development of the hæmal spines as compared with the neural spines of the vertebræ.

Fig. 278.-Tails of different fishes. a Homocercal tail (Sword-fish); b Heterocercal tail (Sturgeon.)

As regards their general distribution in time, the geological history of fishes presents some points of peculiar interest. Of all the classes of the great subkingdom Vertebrata, the fishes are the lowest in point of organisation. It might therefore I have been reasonably expected that they would present us with the first indications of vertebrate life upon the globe; and such is indeed the case. After passing through the enormous group of deposits known as the Laurentian, Huronian, Cambrian, and Lower Silurian formations-representing an immense lapse of time during which, so far as we yet know, no vertebrate animal had been created-we find in the Upper Silurian Rocks the first traces of fish. The earliest of these, in Britain, is found in the base of the Ludlow Rocks (Lower Ludlow Shale), and belongs to the placoganoid genus Pteraspis. Also in the Ludlow Rocks, but at the summit of their upper division, are found fin-spines and shagreen, probably belonging to Cestraciont fishes-that is to say, to fishes of as high a grade of organisation as the Elasmobranchii. So abundant are the remains of fishes in the next great geological epochnamely, the Devonian or Old Red Sandstone-that this period has frequently been designated the "Age of Fishes." Most of the fishes of the Old Red Sandstone belong to the order Ganoidei. In the Carboniferous and Permian Rocks, which close the Paleozoic period, most of the fishes are still Ganoid, but the former contain the remains of many Plagiostomous fishes. At the close of the Paleozoic and the commencement of the

Mesozoic epoch, the Ganoid fishes begin to lose that predominant position which they before occupied, though they continue to be represented through the whole of the Mesozoic and Kainozoic periods up to the present day. The Ganoids, therefore, are an instance of a family which has endured through the greater part of geological time, but which early attained its maximum, and has been slowly dying out ever since. Towards the close of the Mesozoic period (in the Cretaceous period) the great order of the Teleostean or Bony fishes is for the first time known certainly to have made its appearance. The orders of the Marsipobranchii, Pharyngobranchii, and Dipnoi have not left, so far as is known, any traces of their existence in past time. Judging from analogy, however, it is highly probable that the two former of these must have had a vast antiquity, and it is not impossible that the so-called "Conodonts" from the Lower Silurian Rocks of Russia may yet be shown to be the horny teeth of fishes allied to the Lampreys. At present, however, the weight of evidence is in favour of looking upon these problematical little bodies as probably referable to some of the Invertebrata.

These so-called "Conodonts" are microscopic in their dimensions, and have the form of "minute, glistening, slender, conical bodies, hollow at the base, pointed at the end, more or less bent, with sharp opposite margins" (Owen). They show no trace of dental structure, and Professor Owen concludes that they "have most analogy with the spines, or hooklets, or denticles of Naked Molluscs and Annelides."

It is also to be borne in mind that, though it has not yet been possible to definitely refer any fossil fishes to the order of the Dipnoi, recent discoveries have rendered it extremely probable that some well-known extinct types really belong to this order. Thus, the great "Barramunda" (Ceratodus Fosteri of the rivers of Queensland would seem to be truly referable to the Triassic genus Ceratodus, in which case this latter must be removed to the Dipnoi. This remarkable fish also presents some striking points of resemblance with certain extinct Ganoids, such as Dipterus. Upon the whole, therefore, there are good grounds for accepting Dr. Günther's suggestion that the Dipnoi should be regarded as a mere sub-order of the Ganoids.

In the following chapter are given the orders of the Fishes, with the leading characters and geological distribution of each. The order, however, of the Pharyngobranchii (comprising only the living Lancelet), and that of the Marsipobranchii (compris ing the Lampreys and Hag-fishes), may be here dismissed, as

they are not known to be represented by any fossil forms. There remain, therefore, for consideration the orders of the Teleostei (Bony Fishes), Ganoidei (Ganoids), Elasmobranchii (Sharks and Rays), and Dipnoi (Mud-fishes).

ORDER I. TELEOSTEI.-This order includes the great majority of fishes in which there is a well-ossified endoskeleton, and it corresponds very nearly with Cuvier's division of the "osseous" fishes. The Teleostei are defined as follows:-The skeleton is usually well ossified; the cranium is provided with cranial bones, and a mandible is present; whilst the vertebral column almost always consists of more or less completely ossified vertebræ. The pectoral arch has a clavicle; and the two pairs of limbs, when present, are in the form of fins supported by rays. The gills are free, pectinated or tufted in shape, a bony gill-cover and branchiostegal rays being always developed. The branchial artery has its base developed into a bulbus arteriosus; but this is never rhythmically contractile, and is separated from the ventricle by no more than a single row of valves.

The scales in the Teleostean fishes are generally thin, horny, flexible plates, which overlap one another, and have the "cycloid" or "ctenoid" characters. The order, therefore, corresponds, in a general way, with the orders Ctenoidei and Cycloidei of Agassiz. Some of the Teleostean fishes, however, are provided with ganoid scales.

Excluding the Leptolepida, which are sometimes referred to this order, the Teleostei do not seem to have any representatives in times anterior to the Cretaceous period—that is, towards the close of the Mesozoic period. From this time on, however, Bony Fishes with cycloid or ctenoid scales are the chief fossil representatives of the whole class of the fishes, and the order appears to have attained its maximum at the present day.

The order Teleostei is divided into the following sub-orders: SUB-ORDER A. MALACOPTERI, Owen (= Physostomata, Müller).—This sub-order is defined by usually possessing a complete set of fins, supported by rays, all of which are "soft" or many-jointed, with the occasional exception of the first rays in the dorsal and pectoral fins. A swim-bladder is always present,