in forming this class of creatures, discarded the number four, and adopted instead the number six, the great leading idea of the star itself was equally retained in corals of the modern as in those of the more ancient type.
Fig. 47.
Fig. 48.
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Fig. 49. |
Fig. 50. |
Fig. 51. |
The articulata of the Silurian period bore a still more peculiar character. They consisted mainly of the Trilobites—a family in whose nicely-jointed shells the armorer of the middle ages might have found almost all the contrivances of his craft, anticipated, with not a few besides which he had failed to discover; and which, after receiving so immense a development during the middle and later times of the Silurian period, that whole rocks were formed almost exclusively of their remains, gradually died out in the times of the Old Red Sandstone, and disappeared for ever from creation after the Carboniferous Limestone had been deposited. The Palæontologist knows no more unique family than that of the Trilobites, or a family more unlike any which now exists, or a family which marks with more certainty the early rocks in which they occur. And yet, though formed in a fashion that perished myriads of ages ago, how admirably does it not exhibit the articulated type of being, and illustrate that unity of design which, amid endless diversity, pervades all nature. The mollusca of the Silurians ranged from the high cephalopoda, represented in our existing seas by the nautili and the cuttle-fishes, to the low brachipods, some of whose congeners may still be detected in the terebratula of our Highland lochs and bays, and some in the lingulæ of the southern hemisphere. The cephalopods of the system are all of an obsolete type, that disappeared myriads of ages ago—a remark which, with the exceptions just intimated, and perhaps one or two others, applies equally to its brachipods; but of at least two of its intermediate families—the gasteropoda and lamellibranchiata—several of the forms resemble those of recent shells of the temperate latitudes. In its general aspect, however, the Silurian fauna, antiquely fashioned, as I have said, as became its place in the primeval ages of existence, was unlike any other which the world ever saw; and the absence of the vertebrata, or at least the inconspicuous place which they occupied if they were at all present, must have imparted to the whole, as a group, a humble and mediocre character. It seems to have been for many ages together a creation of molluscs, corals, and Crustacea. At length, in an upper bed of the system, immediately under the base of the Old Red Sandstone, the remains of the earliest known fishes appear, blent with what also appears for the first time—the fragmentary remains of a terrestrial vegetation. The rocks beneath this ancient bone-bed have yielded, as I have already said, no trace of any plant higher than the Thallogens, or at least not higher than the Zosteracea—plants whose proper habitat is the sea; but, through an apparently simultaneous advance of the two kingdoms, animal and vegetable—though of course the simultaneousness may be but merely apparent—the first land plants and the first vertebrates appear together in the same deposit.
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
