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. [Illustration: Ichthyosaurus communis.]