Fig. 53

a kind of internal framework; but the latter is unprovided with any external defence, and only has an imperfect skeleton, known in the common cuttlefish as the bone. Of these two groups the former was abundantly present in the ancient seas, as we know by the fragments that remain of its solid stony

NAUTILUS.

habitation. The latter may or may not have been equally abundant, since from the nature of the case its remains could not be so frequently or so well preserved; but we know that various species referrible to it existed throughout the secondary period, and that one genus at least, now extinct, was then extremely common.

The nautilus is a remarkable and a very interesting genus, belonging to the most highly organized, not only of shell-bearing animals, but of all Invertebrata, and even exceeding some of the fishes in this respect.

It is one of the group of Cephalopoda,* or animals whose organs of locomotion are attached immediately around and upon the rim of the mouth or head. The fore part of the body forms a strong and wide sheath, each side of which produces a group of conical processes pierced with openings, through which prehensile organs or feelers are, at the will of the animal, projected or retracted. The sheath forming the base of these organs, which are used both for locomotion and prehension, is also a pouch, in which is

contained the mouth and the eyes; and is, in fact, the head, containing what may be regarded as a true brain. The rest of the animal is immediately contiguous, and includes the stomach and other organs; and all the soft parts are inclosed in what is called a mantle or sac, a thick muscular skin, which keeps them together in their places and enables them to act in concert, but offers no defence against an external enemy.

The animal of the nautilus, thus constructed, having considerable powers of locomotion and a complex organization, was inclosed in a stony habitation somewhat resembling many common univalve shells. But such an appendage adds considerably to the specific gravity of the whole mass which, even without it, could hardly be lighter than water; and with it, unless by some special contrivance, would for ever remain at the bottom of the sea. The contrivance is a very simple one, and consists merely in a provision by which, as the animal grows in size, it from time to time. builds off a cup-shaped wall upon the soft rounded surface of the hinder part of the body, leaving as it goes a space behind it, which is occupied only by air or some gaseous substance, and acts as a float.

Proceeding in this way, and building a succession of these walls, there is ultimately formed what is called a chambered shell; and all the chambers but the outer one, (that containing the animal,) being filled with air, are in a condition to support the body and shell in the water without sinking. A communication is kept up between the sac inclosing the heart (the pericardium) and the various chambers by a tube passing through all the walls in succession; and it

has been supposed, although it is by no means proved, that this tube permits of a change being effected in the specific gravity of the animal, by forcing in a small quantity of fluid in addition to that which the tube generally contains.

The nautilus is a genus which has been almost universally distributed both in time and space, having been introduced at a very early period, existing apparently in all parts of the ancient seas, and continued in some shape or other even up to the present day. The Cephalopoda, as a group, are, however, chiefly characteristic of the first and middle epochs; and one form was greatly developed during the early portion of the period we are now considering. The genus Ammonites, the one here alluded to, may therefore be properly described when speaking of the lias fauna. There are, however, certain subdivisions of the genus as it now stands, which appear confined to beds of a certain age; and the annexed figure represents a group peculiar to the older part of the secondary epoch.

In the true ammonite the shell is spiral, and coiled on itself in one plane, the whorls at least touching, and not unfrequently enclosing one another. It differs from the nautilus in several respects, namely, (1) in the substance of the shell itself, which is generally, though

Fig. 54

not invariably, much thinner AMMONITE OF THE LIAS. than in the latter genus; (2) in (Group Falciferi.)

the form of the aperture and the relative proportion

of the last or outer chamber; (3) in being frequently covered with numerous bosses, tubercles, ribs, and other projections, which greatly ornament the shell, and of whose use we shall presently speak; (4) in the nature of the wall of separation between each two adjacent chambers; and (5) in the position of the siphuncle or tube communicating with the various chambers.

The shell of the ammonite is usually thin, the outer chamber extends for about two-thirds or more of an entire whorl, affording a considerable space for the animal to inhabit; and the external aperture is often provided with very singular projections, or is swelled out, affording, in all probability, a means of attachment, by which the animal was safely inclosed within its shell. But as this part seems to have been generally thin and fragile, it is rarely preserved in ordinary specimens.

To make up, perhaps, for the thinness of the shell, and to give additional strength without greatly increasing the weight, most of the different species of ammonites are ribbed and covered with tubercles, which, if we look upon the shell simply as a mechanical contrivance for defending a soft animal, and consisting of a continuous arch coiled round itself, served the purposes of transverse arches and domes. The external surface was greatly strengthened by such an arrangement, and the nature of the strength thus communicated it is not difficult to understand. The introduction of the ribs, which were distributed over the surface of the shell transversely, corresponds to the introduction of fluted metal instead of a plane surface, often made use of in machinery when it is wished

to combine the greatest resisting power with the smallest weight of material. Additional strength is also gained by the bosses or elevations of part of the ribbed surface into dome-shaped tubercles; for these, like the vaultings in architecture, give strength to the surface to be supported, and are therefore usually placed at those parts of the external shell beneath which there is no immediate support from the internal walls which separate the chambers from one another.

It is, however, chiefly in the arrangement and construction of these walls of

Fig. 55

SEPTUM

or Chamber-wall of an AMMONITE.

separation (fig.55), that mechanical contrivance seems carried to its height in the shell of the ammonite; but here, also, the contrivance is exceedingly simple, and merely consists in causing the extremities of the walls, where they meet the shell, to deviate into a variety of ramifications and undulating lines. This is singularly shewn in the progressive change by which the Nautilus (fig. 53, p. 139), where these lines of intersection are nearly straight, is succeeded by the Clymenia, where they are decidedly curved; then by the Goniatite (fig. 40. p. 96), where they become angular; then again by the Ceratite (fig. 45, p. 119), where they are rounded and exhibit a tendency to undulations; and lastly by the Ammonite, in many species of which the sinuous windings of these sutures, at their union with the external shell, are singularly complicated and beau

*See figs. 102, 104, where are represented closely allied forms, in which the nature of the intersection does not differ from that observed in the most typical ammonites.