The Story of a Boulder; or, Gleanings from the Note-book of a Field Geologist

by Archibald Geikie

[42] See Egerton, Quart. Jour. Geol. Soc., vol. ix. p. 281.

The placoid teeth of the carboniferous rocks show the usual forms of the order. Some of them are sharp and pointed, as those of the hybodonts; others have a smooth, rounded, or plate-like form, as in the cestracionts. The latter often show a dark brilliant surface, and might be readily enough mistaken for well-worn pebbles. In the oblong rounded teeth of psammodus the surface is densely covered with minute points like grains of sand, whence the name of the genus. These teeth, when sliced and viewed under the microscope by transmitted light, exhibit a complex reticulated internal structure.

Agassiz' second great Order of fishes is named Ganoid, from a Greek word signifying brightness, in allusion to the brightly enamelled surface of their dermal covering. They differ from the placoids in having their outer surface cased in a strong armature of bone, which is disposed either in the form of large overlapping plates, as among the strange tortoise-like fishes of the Old Red Sandstone, or as thick scales, which are either placed at intervals, as along the back and sides of the sturgeon, or closely imbricated, as in the stony-gar (lepidosteus) of the American rivers. This strong, massive skeleton constitutes in many genera the sole support of the animal framework, the inner skeleton being of a gristly cartilaginous kind, like that of the skate. On this account traces of the vertebral column are by no means abundant among the older formations. But as the ganoids form a sort of intermediate link between the placoid or gristly fishes on the one hand, and the bony fishes on the other, they are found to present in their different genera examples of both these kinds of structure. Thus, the skeleton of the sturgeon consists of a firm cartilage, out of which the vertebræ are moulded, so that this fish was at one time ranked with the sharks in the cartilaginous tribe of Cuvier. The skeletons of some of the older ganoids (as holoptychius), on the other hand, manifest such a decidedly osseous structure, with sometimes so much of a reptilian cast, that the bones were at first referred to some huge extinct saurians. The head of the ganoid fishes is encased in a set of large massive plates of bone, and the jaws are furnished with several rows of small sharp teeth, intermingled with a less numerous but larger-sized and more formidable kind. The interior of the mouth likewise displayed in many ancient genera groups of palatal teeth, so that the dental apparatus of these animals must have been very complex and complete. The tail in all the older ganoids was heterocercal, like that of the sharks, the lobes being not unfrequently densely covered with minute overlapping scales of bone—a peculiarity which also extended to the fins. But the fins were sometimes strengthened in another way by having the foremost ray greatly thickened and enlarged, so as to form a stiff spine like the ichthyodorulites of the placoids. The whole of the external surface of these ganoidal fishes glittered with enamel, and was usually sculptured in the most graceful patterns or ornamented with fine lines and punctures so minute as to be almost invisible to the naked eye. Every plate, scale, fin-ray, nay, the very lips exhibited the characteristic enamel mottled over with the style of ornament peculiar to the species. And when we think we have exhausted the contemplation of these beauties, it needs but a glance through an ordinary microscope to assure us that the unassisted eye catches only a superficial glimpse of them. The more highly we magnify any portion of these old-world mummies, the more exquisite does its structure appear.

In the carboniferous rocks of Great Britain, upwards of forty species of ganoids have been detected. They have a wide range in size, the smallest measuring scarce two or three inches, while the largest, to judge at least from the bones which they have left behind, must have reached a length of twenty, or perhaps even thirty feet The lesser genera ([Fig. 29]) were characterized by small, angular, glossy scales, usually ornamented either with a very minute punctulation, or with fine hair-like lines which sometimes exhibited the most complicated patterns. The scales were likewise occasionally serrated along the exposed edges—a style of ornament which gives no little richness to the aspect of the dermal covering. The fins, closely imbricated with small angular scales of bone, sometimes displayed a striated ray in front, but this neither possessed the strength nor the formidable aspect of the corresponding spine among the placoids. The head was encased in a set of bony plates fitting tightly into each other, and ornamented with various patterns according to the species. The teeth were very small and fine, resembling the bristles of a brush, but in at least some species intermingled with teeth of a larger size. The minute style of dentition in these smaller fishes has been thought to indicate their habit of keeping to the bottom of the water and feeding on the soft decaying substances lying there. Nowhere have I seen the small rhomboidal scales of the palæoniscus so abundant as among dark shales charged with cypris cases and fragments of terrestrial plants, and on such occasions the idea has often occurred that these graceful little fishes, like the amia of the American rivers, may have fed on the cyprides that swarmed along the bottom of the estuary.

Fig. 29.—Amblypterus macropterus (a Carboniferous ganoid).

Scattered over the fresh-water limestones, ironstones, and shales, or crowded together along the upper surface of some of the coal-seams, there occur the remains of two very remarkable ganoidal fishes. They deserve our attention for their great size, their complex organization, and the important place in the scale of animal life which they occupied during a former period. One of them has been called megalichthys or great fish—an unhappy name, since the animal did not reach the dimensions attained by not a few of the other ganoids, and was even surpassed by at least one of its contemporary congeners. The other is known as the holoptychius or wrinkled scale. A more detailed examination of these two animals will perhaps best enable us to understand the character of the ganoid fishes that lived in the waters of the Carboniferous period.

The megalichthys had an average length of about three feet. Like the other members of the ganoid order it had a glittering exterior, every scale and plate being formed of strong bone, and coated with a bright layer of enamel. Wherever this polished surface extends, it is found to be ornamented with a minute punctulation, the pores of which lie thickly together like the finer dots of a stippled engraving. The cranial plates are further varied by a scattered and irregular series of larger punctures that look as if they had been formed by the insertion of a pin-point into a soft yielding surface. The examination of the head of the megalichthys as depicted in [Fig. 30], will convey an adequate conception of the structure of a ganoidal cranium.

Fig. 30.—Head of Megalichthys Hibberti, one-sixth of natural size (Agass. Poiss. Foss. Tab. 63).
A Upper side. B. Under side. C Profile.