The four orders into which this grand class of vertebrata is divided by M. Agassiz, are founded upon the peculiar structure of the scales;[507] and are characterised as follow:—
[507] For illustrations of the scales of fishes, see Wond. p. 339, Lign. 68, and p. 340, Lign. 69; Foss. Brit. Mus. p. 419; and Ly. fig. 306, figs. 342-347, &c. And for their minute structure, see Prof. Williamson’s important paper in the Phil. Trans. for 1851.
Order I. Placoid (a broad plate).—The skin covered irregularly with enamelled plates, sometimes of a large size, but frequently in small points, as the shagreen on the dermal integument of the Sharks and the tubercles of the Rays. [Lign. 185], fig. 1, a fossil placoidian scale from the skin of a shark, highly magnified.
Order II. Ganoid (splendid, from the brilliant surface of the enamel).—The scales are of an angular form, and composed of plates of horn or bone, covered with a thick layer of enamel; their structure is identical with that of the teeth. The Sturgeon is an example of this order. [Lign. 196], figs. 1, 2, 3, 4, are fossil scales of a ganoidian fish.
Order III. Ctenoid (toothed, or comb-like).—The scales are formed of plates, which are toothed or pectinated on their posterior margin or edge, like a comb. As the plates are superimposed on each other, so that the lowermost always extend beyond the uppermost, their numerous sharp points or teeth render the scales very harsh to the touch. The Perch belongs to this order. [Lign. 185], fig. 3, represents a fossil ctenoidian scale.
Order IV. Cycloid (circular).—The scales are composed of simple laminæ, or plates of bone or horn, without enamel, and have smooth borders; but their external surface is often ornamented with markings. The scales of the lateral line consist of funnels placed one within the other; the contracted part of which, applied against the disk of the scale, forms the tube through which the mucus flows. To this order belong the Mullet, Salmon, and Carp. [Lign. 185], fig. 4, is the scale of a fossil cycloidian fish.
Fins of Fishes.—As the progression of fishes through the water is principally effected by the action of the tail, they have no limbs commonly so called. The instruments for balancing the body, and for assisting progression, are the fins, which are composed of numerous rays that support a membranous expansion; and the number and situation of the fins present various modifications in the different orders and genera.
The fins are named according to the situation they occupy; for example, pectoral, those on each side of the chest, and which correspond to the anterior extremities of other vertebrated animals; dorsal, on the back; ventral, on the belly; caudal, on the tail. (See outlines of Fish, Ligns. [186], [187], [195].) The rays are of two kinds; 1st, the Spinous rays; these consist of a single osseous piece, usually dense and pointed, sometimes flexible and elastic, and divided longitudinally (Ligns. [188], [196]); 2d, Soft or articulated rays, which are composed of numerous small articulations or joints, and divide into branches at their extremities. Many species of fishes have four fins; others six; some but two; and in certain genera they are altogether wanting. In a fossil state the fins are often beautifully preserved; even the soft rays in many of the Tertiary marls and in the Chalk, are found entire, and attached to the body in their natural situation. The large, strong, spinous rays of the dorsal fins of the cartilaginous fishes, as the Sharks and Rays, are generally found detached, or connected only with a few vertebræ; but they are so abundant in some of the Secondary deposits (and in numerous instances they are the only vestiges of extinct species and genera), that they possess great geological interest; they are distinguished by the term Ichthyodorulites (fossil fish-weapons), under which head they will hereafter be described (see [Lign. 188]). The first ray in the dorsal fin of some fishes is protected in front by a double row of enamelled scales, and these often occur in a fossil state (see [Lign. 196], fig. 5).