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no fewer than two hundred species of ammonites-agreeing in certain essential characters, but remarkably differing in others—are enumerated, the specimens found in this vicinity appear to be all of the biplex and triplex corrugated species, though varying from three inches to three feet in diameter; and were formerly known as the snake-stones. Still no specimen is found perfect, the parts around the mouth having been probably too fragile to resist the agitation of the mighty waters; for we have reason to imagine to ourselves a vast ocean sinking several thousand feet, that gradually became silted up to the brim. From the circumstance of the occurrence of ammonites being restricted to certain strata, and probably terminating with the chalk epoch, they are like fragments of an inscription, containing a row of figures indicating the age, but of which we have the conclusion only, without the beginning, thus :
000 000, whence we may infer that the missing figures were millions, billions, or trillions. Yet cognate multilocular or chambered shells, extend through the entire series of fossiliferous formations. Two of the most numerous in our own country being the following
The chambered shells to which the ammonite belongs, may therefore be considered as one of the most varied and widely-spread families of all the petrified testacea. Sir Henry De la Beche's researches have afforded satisfactory evidence that the animal successively and exclusively inhabited the outer whorl of the shell. And when the specimen is full-grown, the margin, if found entire, is seen to be rolled outwardly and thickened, as observable in many recent univalves of various genera : the inhabitant not having till then obtained the power of making that extra deposit. This feature, together with the bosses and even spines that adorn some species of ammonites, sufficiently contradict the old opinion of there having been internal shells, partaking of the osseous duties in supporting the flesh, instead of being outward shells, or cases, protecting the softer parts and vital organs within. Another contemporaneous fossil many-chambered shell, the nautilite, has been frequently confounded with it; both being of a flattened spiral figure, all the circumvolutions of which may be divided by the same horizontal plane. And indeed the two must be deemed in a degree related, the principal distinction being that in the ammonite the place of the siphuncle is always upon the exterior of the transverse whorls, or on their dorsal margin; but in the nautilus that delicate organ of hydraulic adjustment is invariably near the middle, or towards the ventral margin. Still, although the extremes of the two species are readily discriminated from each other, yet the intermediate varieties of the ammonitiform nautili and the nautiliform ammonites can with difficulty be distinguished by the general observer. Among these the nautilus pompilius, or pearly nautilus, affords a truly admirable study, since it actually exists in the present, and is also found in the recent, and the ante-diluvian state,-being the only creature known to have maintained its station through the many supposed bouleversements and cataclysms which the globe has undergone: in more exact words-from still existing, it affords a valuable aid towards revealing the inhabitant of the allied fossil forms, which endured from the Devonian period through the carboniferous group, the liassic formation, the oolites, and the green sands, up to the London clay. How so brittle a fabric was enabled thus to withstand the force and fury of the flood, seems somewhat anomalous at a first glance; but the observant inquirer will soon perceive that, although apparently very tender and fragile, it really combines all the elements of the strength
required by its destiny, as designed by Omniscient fiat. Not only is the hull of the nautilus strongly secured by equivalents to beams and trusses, but, by a marvellous adaptation of the hydraulic powers of its syphon and septa, or internal shelly partitions, it can be floated or sunk as calms or storms may render necessary, at the discretion of the resident cephalopod, and moved by the re-action of water forcibly ejected from its funnel. of action has been beautifully expressed by Lord Byron, in alluding to the frail boat into which Captain Bligh was forced by the mutineers of the ill-fated Bounty :
“ The tender nautilus, who steers his prow,
The sea-born sailor of his shell-canoe,
In conclusion it must be repeated that ammonites exhibit, to attentive examination, a systematic union of buoyancy and strength, being constructed every way upon the principle of a continuous arch, coiled like a helix round itself, with the base of the outer whorls resting upon the crown of the inner ones. The curves extend in all directions, and they are further strengthened by ribs and transverse plates, and air-chambers, so as to offer appropriate resistance to every external pressure. It is clearly ascertained that the inhabitant of these shells was armed with a powerful arrangement of the organs of prehension around the head, and hard mandibles for crushing shells and crustacea, and otherwise procuring food; in illustration of which, Dr. Lee has a large ammonite from a limestone quarry, with a crab's claw in it, which had either been swallowed by the cephalopod, or it might have flowed into the shell with the calcareous deposit when it had been abandoned.
During the time my esteemed friend the Dean of Westminster was engaged on
on his admirable analysis of organic nature for his Bridgewater Treatise, I had, several times, under his hospitable roof at Christ Church, Oxford, the advantage of seeing the means he adopted for comprehending his subject; and, being strongly interested in the mechanical laws and purpose of multilocular shells, I cannot but enrich my page with the eloquent conclusion at which he arrived respecting them :
“On examining the proofs of contrivance and design that pervade the testaceous remains of the family of ammonites, we find, in every species, abundant evidence of minute and peculiar mechanisms, adapting the shell to the double purpose of acting as a float, and of forming a protection to the body of its inhabitant.
“ As the animal increased in bulk, and advanced along the outer chamber of the shell, the spaces left behind it were successively converted into air-chambers, simultaneously increasing the power
of the float. This float, being regulated by a pipe, passing through the whole series of the chambers, formed an hydraulic instrument of peculiar delicacy, by which the animal could, at pleasure, control its ascent to the surface or descent to the bottom of the sea.
“ To creatures that sometimes floated, a thick and heavy shell would have been inapplicable; and, as a thin shell, inclosing air, would be exposed to various and often intense degrees of pressure at the bottom, we find a series of provisions to afford resistance to such pressure, in the mechanical construction both of the external shell and of the internal transverse plates which formed the air chambers. First, the shell is made up of a tube, coiled round itself, and externally convex. Secondly, it is fortified by a series of ribs and vaultings disposed in the form of arches and domes on the convex surface of this tube, and still further adding to its strength. Thirdly, the transverse plates that form the air chambers supply also a continuous succession of supports, extending their ramifications, with many mechanical advantages, beneath those portions of the shell which, being weakest, were most in need of them.
“If the existence of contrivance proves the exercise of mind, and if the higher degrees of perfection in mechanism are proof of more exalted degrees of intellect in the Author from whom they proceeded, the beautiful examples which we find in the petrified remains of these chambered shells afford evidence coeval and co-extensive with the mountains wherein they are entombed, attesting the wisdom in which such exquisite contrivances originated, and setting forth the providence and care of the Creator in regulating the structure of every creature of his hand.”
To return to the Hartwell Basin.
The cast of the extinct bivalve named
trigonia is strongly marked by its tripartite division towards the hinge, though the shell has been completely absorbed by the limestone, leaving a vacuum, which is occasionally partly filled with pyrites. The two species of this once
prevailing type of mollusca, the clavellata and incurva, are easily recognised. There are, moreover, some flattish, fluted bivalves, resembling the pecten camellosus, although the ears are mostly destroyed. To these may be added the casts of several species of the cardium, the mytilus, and the plagiostoma rusticum, together with many others. Among the univalves there is a plentiful
. sprinkle of pleurotomarias, naticas, neritas, turritellas, and others, yet all casts. The black and brilliant scales of the lepidotus minor also abound in little heaps, but rarely in their original position.
On descending to the clay, we find that its fine and close texture has preserved, not only the most delicate shells, but even their nacre, with its many-tinted hues, and in one species the epidermis itself. Here, besides the ostrea deltoidea, shewing the effects of great compression, there are other species also, however difficult it may be to pronounce on all the specimens. The pinna quadrata, with its serrated hinge, is a splendid though rare relic. Dr. Mantell, in his examination of the Hartwell fossils a few years ago, recognised a new species of pinna, coated with an uncommonly beautiful nacre, and related to the lanceolata, but probably the granulata according to the opinion of Professor Edward Forbes, who has kindly examined most of our specimens of this series. These pinnas are generally broken across, and the blade, or thin part, is more rarely found than the quadrangular small end, so that Dr. Lee himself possesses only two complete specimens, both about a foot long, and requiring to be imbedded in a lump of clay, in consequence of their prismatic structure (as in the recent pinna) rendering them very fragile. The little modiola elegans abounds to a great degree.
Then we find some bivalves highly deserving of attention, of a stronger texture, and possessing no nacre, but of a horn-like brown: one of these is an astarte, two or three inches long, which Dr. Mantell distinguishes as the Hartwelliensis, it appearing to be a new species, and thus adding to the forty-six already known of this genus. The cardium striatulum (two-thirds of which is striated concentrically, while the furrows on the remaining portion radiate from one side of the hinge) is also found of a similar brown when high in the clay, but more fragile, and