This formation abounds in the remains of radiata, mollusca, and crustacea,—all of them differing specifically from those of the older secondary strata. The gigantic crinoidea of the carboniferous age have disappeared, succeeded only by a few dwarfish specimens of the apiocrinite and pentacrinite, while the ammonites mark an increase of nearly two hundred species, preserved in the most perfect state in the shales and limestones of the lias and oolite.
1. Ammonites obtusus; 2. Section of Ammonites obtusus, showing the interior chambers and siphuncle; 3. Ammonites nodosus.
The term Ammonite has been bestowed upon this remarkable shell-fish from its fancied resemblance to the curved horn on the head of the statue of Jupiter-Ammon. The spiral form of the shell is divided into several chambers or compartments, all of which are connected by means of an interior tube or siphuncle. It belongs to the order of Cephalopoda, among which are included ancyloceros, belemnites, nautilus, orthoceratite, and other many-chambered shells. Like the nautilus, the ammonite was gifted with a singular apparatus by which it could pursue its instincts either at the bottom or on the surface of the element in which it lived. The organs of motion were arranged round the head (hence the name cephalopoda); and, by the nicely adaptive arrangements of nature, the outer chamber of the shell was capable of retaining the entire body of the animal, while the interior chambers were hollow, thereby rendering the whole structure of nearly the same specific gravity with the waters in which it moved. An elastic tube passing through the siphuncle connected the cavity of the heart with the extremity of the shell, which enabled the animal to contract or expand itself as its exigencies required. Being filled with a dense fluid, excreted by the glandular organs, the creature, when alarmed or wishing to descend, withdrew itself within the outer chamber, whereby the contraction of the tube forced the fluid from the heart into the siphuncle, and increasing the gravity enabled it to descend to the bottom. Upon a reversal of the process—the simple projection of the arms of the head, and the consequent expansion of the body—the ammonite rose with equal facility to the surface, disporting itself at will in its native element. With a view to resist the pressure, when at the bottom, a provision was made by means of the arch-form in the structure of the shell; and, additionally, by the insertion of a series of transverse ribs, which comprise all the mechanical contrivances for giving strength and solidity which are sought by the divisions and subdivisions in the vaulted roofs of our Gothic architecture. The geographical distribution of the ammonite partakes of the universality so marked in the vegetable economy of the carboniferous age, the same species even being common to Europe, Asia, North and South America; and always, along with all its numerous congeners, manifesting the most striking examples of that adaptation of means to ends which prevails in every department of creation.
We shall now select a few details of the more remarkable of the reptilian types of this period, referring the reader to the ample and graphic descriptions of Buckland, Conybeare, Mantell, Phillips, and more especially to the Reports of Professor Owen, in the volumes published by the British Association in 1840-’1.
Ichthyosaurus communis.
1. The first genus to be noticed is termed the Ichthyosaurus, which partakes at once of the characters of crocodiles, lizards, and fishes. So lavish has nature been in providing for the accommodation and wants of this anomalous creature, that to the paddles of a whale, is added the sternum of an ornithorhynchus; the head of a lizard is joined to the vertebræ of a fish; and the snout of a porpoise is combined with the jaws and teeth of a crocodile. The magnitude of the eye is prodigious, and the jaws, sometimes exceeding six feet in length, are studded with an apparatus of teeth, amounting in some instances, to a hundred and eighty. “From the quantity of light admitted in consequence of the prodigious size of the eye,” says Dr. Buckland, “it must have possessed very great powers of vision: we have also evidence that it had both microscopic and telescopic properties. We find on the front of the orbital cavity, in which this eye was lodged,”—a cavity sometimes fourteen inches in diameter,—“a circular series of petrified thin bony plates, ranged round a central aperture, where once was placed the pupil; the form and thickness of each of these plates very much resembles that of the scales of the artichoke. It also tends to associate the animal in which it existed, with the family of lizards, and exclude it from that of fishes.” These bony plates gave strength to the surface of the eye-ball, which required protection above and below, from the dashing of the waves when it reared its head to the storm, and from the pressure of deep water when it scoured the bottom. The nostrils, it would seem, were placed so close to the anterior angle of the eye, as to render it impossible to breathe without raising the organs of sight to the surface of the water. An ocean, peopled with such monsters! Imagine so many eyes, larger than a man’s head studding its surface, and illuminating, as with fire-balls, their terrific jaws, glaring out from the briny flood; and what a scene to gaze upon, so different from all that now covers these rich alluvial plains!
