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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 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.†t If, in all these families, it can be shown 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."

tending to fall flat upon its side; thus exposing itself to injury by fric tion, 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. I have observed that a similar vertical position is maintained by the shell of the Planorbis corneus, whilst in the act of crawling at the bottom.

* The recent observations of Mr. Owen show, 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 Manuel of Geology, enumerates 270 species; these species differ* according to the age of the strata 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 ex. istence 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 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.

which they are found, and vary in size from a line to more than four feet in diameter.*

It is needlees here to speculate either on the physical, or final causes which produced these curious changes of species, in this highest order of the Molluscous inhabitants of the seas, during some of the early and the middle ages of geological chronology; but the exquisite symmetry, beauty, and minute delicacy of structure, that pervade each variation of contrivance throughout several hundred species, leave no room to doubt the exercise of Design and Intelligence in their construction; although we cannot always

[blocks in formation]

"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.

* 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 pres

sure.

† The strata here termed primary are those which, in the Sections, (Pl. 1,) I have included in the lower region of the transition series.

VOL. I.-22

point out the specific uses of each minute variation, in the arrangement of parts fundamentally the same.

The geographical distribution of Ammonites in the ancient world, seems to have partaken of that universality, we find so common in the animals and vegetables of a former condition of our globe, and which differs so remarkably from the varied distribution that prevails among existing forms of organic life. We find, the same genera, and, in a few cases, the same species of Ammonites, in strata, apparently of the same age, not only throughout Europe, but also in distant regions of Asia, and of North and South America.*

Hence we infer that during the Secondary and Transition periods a more general distribution of the same species, than exists at present, prevailed in regions of the world most remotely distant from one another.

An Ammonite, like a Nautilus, is composed of three essential parts: 1st. An external shell, usually of a flat discoidal form, and having its surface strengthened and ornamented with ribs (see Pl. 35, and Pl. 37.) 2d. A series of internal air-chambers formed by transverse plates, inter

* Dr. Gerard has discovered at the elevation of sixteen thousand feet in the Himmalaya Mountains, species of ammonites, c. g. A. Walcoti, and A. communis, identical with those of the Lias at Whitby and Lyme Regis. He has also found in the same parts of the Ilimmalaya, several species of Belemnite, with Terebratulæ and other bivalves, that occur in the English Oolite; thereby establishing the existence of the Lias, and Oolite formations in that elevated and distant region of the world. He has also collected in the same Mountains, shells of the genera Spirifer, Producta, and Terebratula, which occur in the Transition formations of Europe and America.

The Greensand of New Jersey also contains Ammonites mixed with Hamites and Scaphites, as in the greensand of England, and Captain Beechy, and Lieutenant Belcher found Ammonites on the coast of Chili in Lat. 36 S. in the Cliffs near Conception; a fragment of one of these Ammonites is preserved in the Museum of Hasler Hospital at Gosport.

Mr. Sowerby possesses fossil shells from Brazil resembling those of the Inferior Oolite of England.

secting the inner portion of the shell, (See Pl. 36 and 41.) 3d. A siphuncle, or pipe, commencing at the bottom of the outer chamber, and thence passing through the entire series of air-chambers to the innermost extremity of the shell, (see Pl. 36, d. e. f. g. h. i.) In each of these parts, there are evidences of mechanism, and consequently of design, a few of which I shall endeavour briefly to point out.

External Shell.

The use and place of the shells of Ammonites has much perplexed geologists and conchologists. Cuvier and Lamarck, guided by the analogies afforded by the Spirula, supposed them to be internal shells.* There is, however, good reason to believe that they were entirely external, and that the position of the body of the animal within these shells

* The smallness of the outer chamber, or place of lodgment for the animal, is advanced by Cuvier in favour of his opinion that Ammonites, like the Spirula, were internal shells. This reason is probably founded on observations made upon imperfect specimens. The outer chamber of Ammonites is very seldom preserved in a perfect state, but when this happens, it is found to bear at least as large a proportion to the chambered part of the shell, as the outer cell of the N. Pompilius bears to the chambered interior of that shell. It often occupies more than half, (see Pl. 36. a. b. c. d.) and, in some cases, the whole circumference of the out whorl. This open chamber is not thin and feeble, like the long anterior chamber of the Spirula, which is placed within the body of the animal producing this shell; but is nearly of equal thickness with the sides of the close chambers of the Ammonite.

Moreover, the margin of the mature Ammonite is in some species reflected in a kind of scroll, like the thickened margin of the shell of the garden snail, giving to this part a strength which would apparently be needless to an internal shell. (See Pl. 37. Fig. 3. d.)

The presence of spines also in certain species, (as in A. Armatus, A. Sowerbii,) affords a strong argument against the theory of their having been internal shells. These spines which have an obvious use for protection, if placed externally, would seem to have been useless, and perhaps noxious in an internal position, and are without example in any internal structure with which we are acquainted.

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