The Medals of Creation, Volumes 1 and 2 / First Lessons in Geology and the Study of Organic Remains

by Gideon Algernon Mantell

Skeletons of Fishes.—The skeletons of the animals of this class differ so remarkably in their relative degree of firmness and elasticity, in consequence of peculiar modifications of their constituent substance, as to form two grand divisions; one of which is termed the osseous, the other the cartilaginous. The essential difference in the skeletons of these two groups consists in the presence or absence of earthy matter (phosphate and carbonate of lime) in the materials of which they are constructed. In the cartilaginous fishes, the skeleton is cartilaginous and transparent; but in some species, the skin has dense osseous particles or plates on the skin, as in the Rays; and in others, the head and body are protected by large osseous scutcheons, as in the Sturgeon. There is also an intermediate group of fishes, termed the fibro-cartilaginous, in which the skeleton contains lime, but in a much less proportion than in the true osseous fishes. In some genera, certain portions of the skeleton, as the bodies of the vertebra, are cartilaginous, while the spinous processes, ribs, &c. are osseous; these characters are of considerable importance in the investigation of the fossil remains of fishes, as we shall hereafter have occasion to demonstrate.

The skeleton consists of the cranium or skull, which is composed of numerous bones,—the jaws, and bones of the tongue,—the osseous frame-work of the organs of respiration, consisting of the bones, rays, and arches that support the gills, and the opercula, or covers which close over the branchial apertures,—and of the vertebral column, formed of numerous dorsal and caudal vertebræ, with the ribs and other appendages; there are no proper cervical vertebræ, or spinal bones of the neck.

The branchial arches are in general four or five on each side, and are attached above to the cranium, and below to a chain of small bones, by which they are connected with the os hyoides, or bone of the tongue. The opercular bones, composing the cover or lid of the opening of the gills, consist of three pieces on each side, and are distinguished by the names, opercular, pre-opercular, and sub-opercular, according to the situations which they respectively occupy.

The vertebræ are double hollow cones,[514] not unlike an hour-glass in form: the interval between two of these bones is filled up, in the living state, by a gelatinous fluid. Along the upper part of each vertebra, there is an annular cavity, which in the united vertebral column forms a canal for the spinal marrow; the posterior dorsal and caudal vertebæ have also a channel below, for the passage of the large blood-vessels.

[514] There are certain exceptions; thus in the Lepidosteus the vertebral column is a series of ball-and-socket joints, the convexity being anterior, as in the land Salamander, and in the fossil reptile known as the Streptospondylus.

There are likewise bones analogous to some of those which enter into the composition of the extremities, chest or thorax, and pelvis of the higher vertebrata; but which it is not necessary for our present purpose here to describe.

Of the organs of vision some fossil remains also occur. The sclerotic coat, or capsule of the eye, being bony in fishes, is often preserved; and in several chalk specimens I have found it occupying the orbit.

In addition to those durable parts of fishes, already mentioned, as likely to be met with in a fossil state, the bones called otolithes (ear-stones) must be enumerated. These calcareous bodies are found in the membranous labyrinth of the organs of hearing; and, although more or less developed in the ear-bulb of all animals, they are larger and of more definite forms in the higher osseous and cartilaginous fishes. The otolithes are supposed to assist in communicating more vivid impressions of sounds to the extremities of the auditory nerves; they are stony in most aquatic animals, and friable or pulverulent in those that live on land. Smooth, oblong otolithes are not uncommon in the Crag deposits of Norfolk and Suffolk; and minute ear-bones are found in the Barton Clay.

Tails of Fishes.—The tail, as we have previously mentioned, is the chief instrument of progressive motion in these animals; it assumes two principal modifications. In the greater number of the existing species the vertebral column terminates in a triangular plate of bone (formed by the fusion of the last few vertebræ), to which the 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. In the second modification the vertebral column towards its extremity diverges from a straight line, rises up, and is prolonged into the upper lobe of the tail; the caudal fin appearing like a rudder, and its low’er lobe, being destitute of vertebræ, is proportionably very feeble and small, as in the Shark and Dog-fish: this form of tail is called heterocercal, i. e. unequal-tail (see Foss. Brit. Mus. p. 421; and Ly. figs. 340, 341). In the embryonic state the tail in all fishes is heterocercal, and it becomes homocercal in the progress of development in those genera which have this type of the caudal appendage. But few of the existing species have the heterocercal tail, while it is found in all the fossil fishes that occur in the ancient secondary strata; namely, the Magnesian limestone, and antecedent deposits. The rounded and equal-bilobed, or homocercal, tails, are seen in the fishes from the Chalk, Wond. pp. 347, &c.; and in the Wealden Lepidotus, Lign. 186; and the unequal or heterocercal tail is shown in the Amblypterus from the Carboniferous strata, [Lign. 187].