What, let us inquire, is the character of these ancient fishes, that first complete the scale of animated nature in its four master ideas, by adding the vertebrate to the invertebrate divisions? So far as is yet known, they all consist of one well marked order—that placoidal order of Agassiz that to an internal framework of cartilage adds an external armature, consisting of plates, spines, and shagreen points of solid bone. Either of the two kinds of dog-fishes on our coasts—the spiked or spotted—maybe accepted as not inadequate representatives of this order as it now exists. The Port Jackson shark, however—a creature that to the dorsal spines and shagreen-covered skin of the common dog-fish adds a mouth terminal at the snout, not placed beneath, as in most other sharks, and a palate covered with a dense pavement of crushing teeth—better illustrates the order as it first appeared in creation than any of our British placoids.
Fig. 52.
Fig. 53.[13]
Fig. 54.
Fig. 55.
Fig. 56.
And here let me adduce another and very remarkable instance of the correspondence which obtains between the sequence in which certain classes of organisms were first ushered into being, and the order of classification adopted, after many revisions, by the higher naturalists. Cuvier, with not a few of the ichthyologists who preceded him, arranged the fishes into two distinct series—the Cartilaginous and Osseous; and these last he mainly divided into the hard or spiny-finned fishes, and the soft or joint-finned fishes. He placed the sturgeon in his Cartilaginous series; while in his soft-finned order he found a place for the Polypterus of the Nile and the Lepidosteus of the Ohio and St. Lawrence. But the arrangement, though it seemed at the time one of the best and most natural possible, failed to meet any corresponding arrangement in the course of geologic history. The place assigned to the class of fishes as a whole corresponded to their place in the Palæontological scale;— first of the vertebrate division in the order of their appearance, they border, as in the "Animal Kingdom" of the naturalist, on the invertebrate divisions. But it was not until the new classification of Agassiz had ranged them after a different fashion that the correspondence became complete in all its parts. First, he erected the fishes that to an internal cartilaginous skeleton unite an external armature of plates and points of bone, into his Placoid order; next, gathering together a mere handful of individuals from among the various orders and families over which they had been scattered—the sturgeons from among the cartilaginous fishes, and the lepidosteus and polypterus from among the Clupia or herrings—he erected into a small ganoid order all the fishes that are covered, whatever the consistency of their skeleton, by a continuous or nearly continuous armor of enamelled bone, or by great bony plates that lock into each other at their edges. Out of the remaining fishes—those covered with scales of a horny substance, and which now comprise nearly nine tenths of the whole class—he erected two orders more—a Ctenoid order, consisting of fishes whose scales, like those of the perch, are pectinated at their lower edges like the teeth of a comb, and a Cycloid order, composed of fishes whose scales, like those of the salmon, are defined all around by a simple continuous margin; and no sooner was the division effected than it was found to cast a singularly clear light on the early history of the class. The earliest fishes—firstborn of their family—seem to have been all placoids. The Silurian System has not yet afforded trace of any other vertebral animal. With the Old Red Sandstone the ganoids were ushered upon the scene in amazing abundance; and for untold ages, comprising mayhap millions of years, the entire ichthyic class consisted, so far as is yet known, of but these two orders. During the times of the Old Red Sandstone, of the Carboniferous, of the Permian, of the Triassic, and of the Oolitic Systems, all fishes, though apparently as numerous individually as they are now, were comprised in the ganoidal and placoidal orders. The period of these orders seems to have been nearly correspondent with the reign, in the vegetable kingdom, of the Acrogens and Gymnogens, with the intermediate classes, their allies. At length, during the ages of the Chalk, the Cycloids and Ctenoids were ushered in, and were gradually developed in creation until the human period, in which they seem to have reached their culminating point, and now many times exceed in number and importance all other fishes. We do not see a sturgeon (our British representative of the ganoids) once in a twelvemonth; and though the skate and dog-fish (our representatives of the placoids) are greatly less rare, their number bears but a small proportion to that of the fishes belonging to the two prevailing orders, of which thousands of boat-loads are landed on our coasts every day.
The all but entire disappearance of the ganoids from creation is surely a curious and not unsuggestive circumstance. In the human family there are races that have long since reached their culminating point, and are now either fast disappearing or have already disappeared. The Aztecs of Central America, or the Copts of the valley of the Nile, are but the inconsiderable fragments of once mighty nations, memorials of whose greatness live in the vast sepulchral mounds of the far West, or in the temples of Thebes or Luxor, or the pyramids of Gizah. But in the rivers of these very countries—in the Polypterus of the Nile, or the Lepidosteus of the Mississippi—we are presented with the few surviving fragments of a dynasty compared with which that of Egypt or of Central America occupied but an exceedingly small portion of either space or time. The dynasty of the ganoids was at one time coextensive with every river, lake, and sea, and endured during the unreckoned eons which extended from the times of the Lower Old Red Sandstone until those of the Chalk. I may here mention, that as there are orders of plants, such as the Rosaceæ and the Grasses, that scarce preceded man in their appearance, so there are families of fishes that seem peculiarly to belong to the human period. Of these, there is a family very familiar on our coasts, and which, though it furnishes none of our higher ichthyic luxuries, is remarkable for the numbers of the human family which it provides with a wholesome and palatable food. The delicate Salmonidæ and the Pleuronectidæ—families to which the salmon and turbot belong—were ushered into being as early as the times of the Chalk; but the Gadidæ or cod family—that family to which the cod proper, the haddock, the dorse, the whiting, the coal-fish, the pollock, the hake, the torsk, and the ling belong, with many other useful and wholesome species—did not precede man by at least any period of time appreciable to the geologist. No trace of the family has yet been detected in even the Tertiary rocks.
Fig. 57.
Of the ganoids of the second age of vertebrate existence—that of the Old Red Sandstone—some were remarkable for the strangeness of their forms, and some for constituting links of connection which no longer exist in nature, between the ganoid and placoid orders. The Acanth family, which ceased with the Coal Measures, was characterized, especially in its Old Red species, by a combination of traits common to both orders; and among the extremer forms, in which Palæontologists for a time failed to detect that of the fish at all, we reckon those of the genera Coccosteus, Pterichthys, and Cephalaspis. The more aberrant genera, however, even while they consisted each of several species, were comparatively short lived. The Coccosteus and Cephalaspis were restricted to but one formation apiece; while the Pterichthys, which appears for the first time in the lower deposits of the Old Red Sandstone, becomes extinct at its close. On the other hand, some of the genera that exemplified the general type of their class