minates in the smallest chamber at the inner extremity of the shell. I shall presently attempt to show how by means of a peculiar fluid, admitted into or abstracted from this pipe, the animal has the the power to increase or diminish its specific gravity, and to sink or float accordingly; as the floating portion of that beautiful toy the Water balloon is made to descend or ascend by means of water forced into, or abstracted from its interior. (See P. 248.)

The motion of the Nautilus, when swimming, with its arms expanded, is retrograde, like that of the naked Cuttle Fish, being produced by the reaction of water, violently ejected from the funnel (k.)

The position assumed during this operation is that which is best adapted to facilitate its passage through the water, as it places foremost that portion of the shell, which approaches most nearly in form to the prow of a boat. The fingers and tentacula (p, p,) are here represented as closed around the beak, which is consequently invisible; when the animal is in action, they are probably spread forth like the expanded rays of the sea Anemone.

The horny beak of this recent Nautilus (See Pl. 31, Fig. 2, 3) resembles the bill of a Parrot. Each mandible is armed in front, with a hard and indented calcareous point, adapted to the office of crushing shells and crustaceous animals, of which latter, many fragments were found in the stomach of the individual here represented. As these belonged to species of hairy brachyurous crustacea, that live exclusively at the bottom of the sea, they show that this Nautilus, though occasionally foraging at the surface, obtains part of its food from the bottom. As it also had a gizzard, much resembling that of a fowl, we see in this organ, farther evidence that the existing Nautilus has the power of digesting hard shells.*

In Pl. 31, Fig. 3 represents the lower mandible, armed in front like Fig. 2. with a hard and calcareous margin; and Fig. 4 represents the anterior calcareous part of the palate of the upper mandible Fig. 2. formed of the VOL. I.-21

A similar apparatus is shown to have existed in the beaks. of the inhabitants of many species of fossil Nautili, and Ammonites, by the abundance of fossil bodies called Rhyncholites, or beak-stones, in many strata that contain these fossil shells, e. g. in the Oolite of Stonesfield, in the Lias at Lyme Regis and Bath, and in the Muschelkalk at Luneville.

As we are warranted in drawing conclusions from the structure of the teeth in quadrupeds, and of the beak in birds, as to the nature of the food on which they are respectively destined to feed, so we may conclude, from the resemblance of the fossil beaks, or Rhyncholites, (Pl. 31, Fig. 5—11,) to the calcareous portions of the beak of the Cephalopod, inhabiting the N. Pompilius, that many of these Rhyncolites were the beaks of the cephalopodous inhabitants of the fossil shells with which they are associated; and that these Cephalopods performed the same office in restraining excessive increase among the Crustaceous and Testaceous inhabitants of the bottom of the Transition and Secondary seas, that is now discharged by the living Nautili, in conjunction with the carnivorous Trachelipods.*

Assuming, therefore, on the evidence of these analogies, that the inhabitants of the shells of the fossil Nautili and Ammonites were Cephalopods, of similar habits to those of the animal which constructs the shell of the N. Pompilius, we shall next endeavour to illustrate, by the organization and habits of the living Nautilus, the manner in which these fossil shells were adapted to the use of creatures, that some

same hard calcareous substance at its point; this substance is of the nature of shell.

These calcareous extremities of both mandibles are of sufficient strength to break through the coverings of Crustacea and shells, and as they are placed at the extremity of a beak composed of thin and tough horn, the power of this organ is thereby materially increased.

In examining the contents of the stomach of the Sepia vulgaris, and Loligo, I have found them to contain numerous shells of small Conchifera.

* See p. 192.

times moved and fed at the bottom of deep seas, and at other times rose and floated upon the surface.

The Nautili (see Pl. 31. Fig. 1. and Pl. 32. Figs. 1. 2.) constitute a natural genus of spiral discoidal shells, divided internally into a series of chambers that are separated from each other by transverse plates; these plates are perforated to admit the passage of a membranous tube or siphuncle either through their centre, or near their internal margin. (Pl. 1. Fig. 31. Pl. 32. Fig. 2. and Pl. 33.)

The external open chamber is very large, and forms the receptable of the body of the animal. The internal close chambers contain only air, and have no communication with the outer chamber, excepting by one small aperture in each plate for the passage of a membranous tube, which descends through the entire series of plates to the innermost extremity of the shell, (Pl. 31, y. y. a. b. c. d. e. and Pl. 32, a. b. d. e. f.) These air-chambers are destined to counterbalance the weight of the shell, and thereby to render the body and shell together so nearly of the weight of water, that the difference arising from the siphuncle being either empty, or filled with a fluid, may cause the animal to swim or sink.*

*The siphuncle represented in Pl. 31, Fig. 1, illustrates the structure and uses of that organ; in the smallest whorls, from d. inwards, it is enclosed by a thin calcareous covering, or sheath, of so soft a nature as to be readily scraped off by the point of a quill; this sheath may admit of expansion or contraction, together with the membranous tube enclosed within it. In the fossil Nautili, a similar calcareous sheath is often preserved, as in Pl. 32, Figs. 2, 3, and Pl. 33, and forms a connected series of tubes of carbonate of lime, closely fitted to the collar of each transverse plate. In four chambers of the recent shell (Pl. 31, Fig. 1, a. b. c. d.) this sheath is partially removed from the desiccated membranous pipe within it, which has assumed the condition of a black elastic substance, resembling the black continuous siphuncular pipe that is frequently preserved in a carbonaceous state in fossil Ammonites.

At that part of each transverse plate, which is perforated for the passage of the siphuncle, (Pl. 31, Fig. 1, y. y.,) a portion of its shelly matter projects inwards to about one-fourth of the distance across each cham

As neither the siphuncle, nor the external shell have any kind of aperture through which a fluid could pass into the close chambers, it follows that these chambers contain nothing more than air, and must consequently be exposed to great pressure when at the bottom of the sea. Several con

trivances are therefore introduced to fortify them against this pressure.

First, the circumference of the external shell, is constructed every way upon the principles of an Arch, (see Pl. 31, Fig. 1, and Pl. 32, Fig. 1.,) so as to offer in all directions the greatest resistance to any pressure that tends to force it inwards.

Secondly, this arch is farther fortified by the addition of numerous minute Ribs, which are beautifully marked in the fossil specimens represented at Pl. 32, Fig. 1. In this fossil the external shell exhibits fine wavy lines of growth, which, though individually small and feeble, are collectively of

ber, and forms a collar, around the membranous pipe, thus, directing its passage through the transverse plates, and also affording to it, when dis. tended with fluid, a strong support at each collar. A similar projecting collar is seen in the transverse plate of a fossil Nautilus. (Pl. 32, Fig. 2, c, and Fig. 3, e, i. and Pl. 33.) A succession of such supports placed at short intervals from one another, divides this long and thin membranaceous tube, when distended, into a series of short compartments, or small oval sacs, each sac communicating with the adjacent sacs by a contracted aperture or neck at both its ends, and being firmly supported around this neck by the collar of each transverse plate. (See Pl. 32, Figs. 2, 3, and Pl. 33.)

The strength of each sac is thus increased by the shortness of the distance between its two extremities, and the entire pipe, thus subdivided into thirty or forty distinct compartments, derives from every subdivision an ac cession of power to sustain the pressure of any fluid that may be introduced to its interior.

* We learn from Mr. Owen, that there was no possibility of the access of water to the air chambers between the exterior of the siphuncle and the siphonic apertures of the transverse plates; because the entire circumference of the mantle in which the siphuncle originates, is firmly attached to the shell by a horny girdle, impenetrable by any fluid.-Memoir on Nautilus Pompilius, p. 47.

much avail as ribs to increase the aggregate amount of strength. (See Pl. 32, Fig 1. a. to b.)

Thirdly, the arch is rendered still stronger by the disposition of the edges of the internal Transverse plates, nearly at right angles to the sides of the external shell, (See Pl. 32, Fig. 1, b. to c.) The course of the edges of these transverse plates beneath the ribs of the outer shell is so directed, that they act as cross braces, or spanners, to fortify the sides of the shell against the inward pressure of deep water. This contrivance is analogous to that adopted in fortifying a ship for voyages in the Arctic Seas, against the pressure of ice-bergs, by the introduction of an extraordinary number of transverse beams and bulk heads.*

We may next notice a fourth contrivance by which the apparatus that gives the shell its power of floating, is progressively enlarged in due proportion to the increasing weight and bulk of the body of the animal, and of the external chamber in which it resides; this is effected by successive additions of new transverse Plates across the bottom of the outer chamber, thus converting into air-chambers that part of the shell, which had become too small to hold the body. This operation, repeated at intervals in due propor

The disposition of the curvatures of the transverse ribs, or lines of growth, in a different direction from the curvatures of the internal transverse plates, affords an example of farther contrivance for producing strength in the shells both of recent and fossil Nautili. As the internal transverse plates are convex inwards, (see Pl. 32, Fig. 1, b. to c.) whilst the ribs of the outer shell are in the greater part of their course convex outwards, these ribs intersect the curved edges of the transverse plates at many points, and thus divide them into a series of curvilinear parallelograms; the two shorter sides of each parallelogram being formed by the edges of transverse plates, whilst its two longer sides are formed by segments of the external ribs. The same principle of construction here represented in our plate of Nautilus hexagonus, extends to other species of the family of Nautilus, in many of which the ribs are more minute; it is also applied in other families of fossil chambered shells; e. g. the Ammonites, Pl. 35, and Pl. 38. Scaphites, Pl. 44, Fig. 15. Hamites, Pl. 44, Fig. 8-13. Turrilites, Pl. 44, Fig. 14, and Baculites, Pl. 44, Fig. 5.