consist of the mineralized osseous skeleton. Deposits of mud or fine detritus, of whatever age, appear to have been most favourable for the preservation of the dermal integuments; hence we often find in the pulverulent clays and marls of the Tertiary strata, and in the Chalk of England and Westphalia, and in the fine lithographic stone of Solenhofen, fishes, perfect in form, and not only individuals, but groups, with the scales, fins, head, teeth, and even the capsule of the eye, in their original juxtaposition. In Chalk, some of the fishes occur with the body uncompressed, and as entire, as if the original had been surrounded by soft plaster of Paris while floating in the water. But in coarse limestones and conglomerates-in other words, in materials that have been subjected to the action of the waves and torrents-detached teeth, scales, bones, &c. constitute the principal vestiges of this class of beings.1 The cartilaginous or osseous nature of the skeleton, and the number and position of the fins, were the characters formerly employed in the classification of Fishes; but M. Agassiz, conceiving the structure of the skin to afford a natural index to the essential modifications of organization and functions, with great sagacity adopted an arrangement founded upon the form and structure of the scales, and divided the whole class into four orders, each distinguished by essential differences in the dermal (skin) system. To the geologist this method has proved of inestimable value; for it is simple, easy of application, and, so far as our present knowledge extends, may be relied upon as affording accurate conclusions as to the nature and relations of the originals, to which a few detached fossil scales may have belonged. Another important aid has been derived from the microscopical examination of the structure of the teeth; a department of palæontological investigation, which is yet but partially explored. The living species of Fishes exceed eight thousand, and those found in a fossil state, and determined by M. Agassiz, already amount to upwards of two thousand, while several hundreds are still undescribed, and the rapid progress of geological research is continually adding to the number. The 1 See " Medals of Creation," chap. xv. British species form a large proportion of the Ichthyolites illustrated and described by M. Agassiz; and fortunately, in our eminent Ichthyologist, Sir Philip Grey Egerton, Bart. we have an able and zealous cultivator of this branch of Palæontology.1 AGASSIZ'S CLASSIFICATION OF FISHES.-The four orders into which the class PISCES is divided by M. Agassiz, are founded on the following characters :— Order I. PLACOID (from πλà§, 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. 82, fig. 4, a dermal spine from the skin of a Ray. Order II. GANOID (yávos, splendour, 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, a structure which is identical with that of their teeth. The Sturgeon is a living example of this order. Lign. 82, fig. 3, is a fossil scale of a fish of this division, the Lepidotus. Order III. CTENOID (KTels, a comb). -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. 82, fig. 1, represents a fossil ctenoid scale. 3 2 4 LIGN. 82. ILLUSTRATION OF THE SCALES IN THE FOUR ORDERS OF FISHES. 1. CTENOID. 2. CYCLOID. Order IV. CYCLOID (KÚKλOS, circle). 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, The splendid and most important work entitled "Recherches sur les Poissons Fossiles, par LOUIS AGASSIZ," stands pre-eminent in this department of science. It consists of five volumes, 4to. of letter-press, and five volumes folio, of coloured plates. It is the classical work in this branch of natural history, and must be consulted by every one interested in the subject. I would refer the reader for a condensed view of Fossil Ichthyology to my " Medals of Creation," vol. ii. chap. xv. 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. 82, fig. 2, is the scale of a fossil Cycloid 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 properly 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 spinous rays of the dorsal fins of the cartilaginous fishes, as the Sharks and Rays, generally occur detached from the body in a fossil state; they are abundant in some of the secondary deposits, and being often the only vestiges of extinct species and genera, possess great geological interest; they are termed Ichthyodorulites (fossil dorsal-rays of fish). TEETH OF FISHES. Of the durable parts of animals which occur in the mineral kingdom, the teeth of Fishes present the most numerous, varied, and striking modifications of form, structure, composition, mode of arrangement, and attachment; and yet these dental organs, separately considered, in many instances fail to afford characters by which the natural affinities of the original can be satisfactorily ascertained; and without the aid of other parts of the skeleton, it is often impossible to determine whether an unknown form of tooth belonged to an animal of the class of Fishes, or of Reptiles. Although the modifications of form are almost innumerable, they are referable to four principal types; namely, the conical, the flattened, the prismatic, and the cylindrical.1 STRUCTURE OF THE TAIL.-The tail, as we have previously mentioned, is the chief instrument of progressive motion in these animals; it assumes two principal modifications; and these characters the sagacity of M. Agassiz has invested with a high degree of paleontological interest. In the greater number of the existing species, the vertebral column terminates in a triangular plate of bone, to which the 1 See "Medals of Creation," pp. 597–601. caudal fin is attached symmetrically; and its figure is either rounded, or divided into two equal lobes or branches; these tails are termed homocercal, i. e. even-tail, as is shown in Lign. $3, fig. 2. LIGN. 83. ILLUSTRATION OF THE STRUCTURE OF THE TAIL IN FISHES. 1. Heterocercal tail; a, b, the vertebral column extending into the 2. Homocercal tail; of the ANGMARSET (Mallotus villosus) of Green- In the second modification, the vertebral column, towards its extremity, diverges from a straight line, and rises up, and is prolonged into the upper lobe of the tail; the caudal fin appearing like a rudder, and the lower lobe being proportionably very feeble and small, as in the Shark and Dog-fish; (Lign. 83, fig. 1:) this form of tail is called heterocercal, i. e. unequal-tail. But few of the existing fishes have this condition of the caudal fin, while it is found in all the fossils that occur in the ancient secondary strata; namely, the Magnesian limestone, and antecedent deposits. The rounded, and equallybilobed or homocercal tails, are seen in many of the beautiful fishes from the Chalk (Lign. 95); and the rudder-like, or heterocercal tail, is shown in many of the Ichthyolites from the Carboniferous strata (Lign. 85). GANOID ORDER.-The fishes of this order are distinguished by their brilliant angular scales, composed of osseous or corneous plates covered with a dense coat of enamel, which form a dermal integument of great strength and solidity. In many of the most ancient types, the body is literally enveloped in an osseous case; the bones of the cranium coalescing, and the scales of the thoracic, dorsal, and abdominal regions, blending as it were into a cuirass; hence the fishes of this order, which are among the most ancient known types of vertebrata, appearing in the Devonian or Old Red formation, afford the only absolute knowledge we possess of the earliest forms and structures of Ichthyic organization; for of the Placoids, which appear in a still earlier geological epoch,-namely, the Silurian, -owing to the cartilaginous and perishable nature of their skeletons, a few rays or spines, teeth, scutcheons, and shagreen skins, are the only vestiges that remain in a fossil state. Thus the minutely dentated fin-ray of the Homocanthus, of the Devonian formation, is the only fossil relic of that placoid; while in its contemporary ganoid fish, the Osteolepis, indications of the structure of the organs of smell, hearing, and vision, are manifest. The Ganoid order comprises twelve families, examples of which are arranged in Wall-cases A and B, and are comprised in the subdivisions marked 1 to 16 in the compartments of the glass-cases; as enumerated in the list, ante, p. 416. CEPHALASPIDIANS. Wall-case A. [1.]· The Devonian Formation, (see Geological Table, ante, p. 5,) in which, but twenty-five years ago, a few single scales, discovered in Forfarshire by Dr. Fleming, were the only known traces of any vertebrated animals, has yielded upwards of sixty species. belonging to nearly thirty genera, from British localities alone. Of these the most characteristic are the Cephalaspis, Pterichthys, and Coccosteus, which form a group of extinct genera that has no representative either in the Silurian system below, or in the Carboniferous above; nor, except by distant and faint analogies with existing fishes, can these anomalous organisms be brought within the pale of zoological arrangement. These ichthyolites agree in one general character, that of being covered by relatively enormous osseous or horny plates, and scutcheons. No vertebræ have been found, and it is supposed 1 See "Medals of Creation," p. 645; "Wonders of Geology," p. 760. |