Thirdly, It remains to consider the effect of the air (supposing it to be retained continually within the chambers,) at the bottom of the sea. Here, if the position of the moving animal be beneath the mouth of the shell, like that of a snail as it crawls along the ground, the air within the chambers would maintain the shell, buoyant, and floating at ease above the body; and the

The air within each chamber remains under compression, as long as the siphuncle continues distended by the pericardial fluid; and returning, by its elasticity, to its former state, as soon as the pressure of the body is withdrawn from the pericardium, cooperates with the muscular coat of the siphuncle, to force the fluid back again into the pericardium; and the shell, thus diminished as to its specific gravity, has a tendency to rise.

The place of the pericardial fluid, therefore, will be always in the pericardium, excepting when it is forced into and retained in the siphuncle by pressure of the body on the pericardial sac, during the contraction of the animal within its shell. When the arms and body are expanded, either on the surface, or at the bottom of the sea, the water will have access to the branchial chambers, and the movements of the heart proceed freely in the distended pericardium; which will have great part of its fluid withdrawn at those times only, when the body is contracted into the shell, and the access of water to the branchiæ consequently impeded.

The following experiments shew that the weight of fluid requisite to be added to the shell of a Nautilus, in order to make it sink, is about half an ounce.

I took two perfect shells of a Nautilus Pompilius, each weighing about six ounces and a half in air, and measuring about seven inches across their largest diameter; and having stopped the Siphuncle with wax, I found that each shell, when placed in fresh water, required the weight of a few grains more than an ounce to make it sink. As the shell, when attached to the living animal, was probably a quarter of an ounce heavier than these dry dead

tendency of the shell to rise to the surface would be counteracted by the strong muscular disk (Pl. 31, n.), with which the creature crawls, and adheres to the bottom, using freely its tentacula to seize its prey.*

Dr. Hook considered (Hook's Experiments, 8vo. 1726, page 308) that the air chambers were filled alternately with air or water;† and Parkinson (Organic Remains, vol. iii. p. 102), admitting that these chambers were not accessible to water, thinks that the act of rising or sinking depends on the alternate introduction of air or water into the siphuncle; but he is at a loss to find the source from which this air could be obtained at the bottom of the sea, or to

shells, and the specific gravity of the body of the animal, when contracted into the shell, may have exceeded that of water to the amount of another quarter of an ounce, there remains about half an ounce for the weight of fluid, which being introduced into the siphuncle, would cause the shell to sink; and this quantity seems well proportioned to the capacity both of the pericardium, and of the distended siphuncle.

*. See Sup. Note.

+ If the chambers were filled with water, the shell could not be thus suspended without muscular exertion, and instead of being poised vertically over the body, in a position of ease and safety, would be continually tending to fall flat upon its side; thus exposing itself to injury by friction, and the animal to attacks from its enemies. Rumphius states, that at the bottom, He creeps with his boat above him, and with his head and barbs (tentacula) on the ground, making a tolerably quick progress. The author has observed that a similar vertical position is maintained by the shell of the Planorbis corneus, whilst the animal is in the act of crawling at the bottom.

explain "in what manner the animal effected those modifications of the tube and its contained air, on which the variation of its buoyancy depended."* The theory which supposes the chambers of the shell to be permanently filled with air alone, and the siphuncle to be the organ which regulates the rising or sinking of the animal, by changing the place of the pericardial fluid, seems adequate to satisfy every hydraulic condition of a Problem that has hitherto received no satisfactory solution.

I have dwelt thus long upon this subject, on account of its importance, in explaining the complex structure, and hitherto imperfectly understood functions, of all the numerous and widely disseminated families of fossil chambered shells, that possessed siphunculi.† If, in all these families, it can be shewn that the same principles of mechanism, under various modifications, have prevailed from the first commencement of organic life unto the present hour, we can hardly avoid the conclusion which would refer such unity of organizations to the will and agency of one and the same intelligent First Cause, and lead us to regard them all as "emanations of that Infinite Wisdom, that appears in the shape and structure of all other created beings."+

*The recent observations of Mr. Owen shew, that there is no gland connected with the siphuncle, similar to that which is supposed to secrete air in the air-bladder of fishes.

+ See Sup. Note.

Dr. Hook's Experiments, p. 306.

SECTION IV.

AMMONITES.

HAVING entered thus largely into the history of the Mechanism of the shells of Nautili, we have hereby prepared ourselves for the consideration of that of the kindred family of Ammonites, in which all the essential parts are so similar in principle to those of the shells of Nautili, as to leave no doubt that they were subservient to a like purpose in the economy of the numerous extinct species of Cephalopodous Mollusks, from which these Ammonites have been derived.

Geological Distribution of Ammonites.

The family of Ammonites extends through the entire series of the fossiliferous Formations, from the Transition strata to the Chalk inclusive. M. Brochant, in his Translation of De la Beche's Manual of Geology, enumerates 270 species; these species differ according to the age of the strata in which they are found,† and vary in

+ Thus one of the first forms under which this family appeared, the Ammonites Henslowi, (Pl. 40, Fig. 1), ceased with the Transition formation; the A. Nodosus (Pl. 40, Figs. 4, 5.) began and terminated its period of existence with the MuschelKalk. Other genera and species of Ammonites, in like manner, begin and end with certain definite strata, in the Oolitic and Cretaceous_formations; e. g. the A. Bucklandi (Pl. 37, Fig. 6.) is

size from a line to more than four feet in diameter.*

peculiar to the Lias; the A. Goodhalli to the Greensand; and the A. Rusticus to the Chalk. There are few, if any, species which extend through the whole of the Secondary periods, or which have passed into the Secondary, from the Transition period.

The following Tabular Arrangement of the distribution of Ammonites, in different geological formations, is given by Professor Phillips in his Guide to Geology, 1834, p. 77.

SUB-GENERA OF AMMONITES.

Total, 223 species.

"It is easy to see how important, in questions concerning the relative antiquity of stratified rocks, is a knowledge of Ammonites, since whole sections of them are characteristic of certain systems of rocks."-Phillips's Guide to Geology, 8vo. 1834, sec. 82.

The strata here termed primary are those which, in the Section, (Pl. I), I have included in the lower region of the transition series.

Mr. Sowerby (Min. Conch. vol. iv. p. 79 and p. 81,) and Mr. Mantell speak of Ammonites in Chalk, having a diameter of three feet. Sir T. Harvey, and Mr. Keith Milnes, have recently measured Ammonites in the Chalk near Margate, which exceeded four feet in diameter; and this in cases where the diameter can have been in a very small degree enlarged by pressure.