The classification above sketched is very far from perfect. It is difficult to define the groups, when we pass from the typical to the ordinary species, and there are many forms which refuse to be included in the formula. Still it is an admirable sketch, and when the Ammoniticlw are fully developed, according to the principles thus exemplified by You Buch, we shall have them recognised, not as a genus with subdivisions, but as a family including many genera. (D'Orbigny's Palfeontologie Francaise;"Annales des Sci. Nat.,' 1841, N.S., xvi. p. 113, also (1829) xvii. 267 ; xviii. 417; xxix. 5.) Having given this sketch,. it will be necessary to meet the question whether the Ammonites were external or internal shells. Cuvier and Lamarck thought that they were internaL The former says (` Regne Animal,' last edition), "The smallness of the last chamber might induce us to believe that, like the Spirula, they were internal shells." Mr. Owen, in his arrangement above quoted, says, "Animal unknown, presumed to resemble the Nautilus; shell external . . . . The last chamber the largest and lodging the animal ;" and probably this was the actual state of things. Dr. Buckland, in his Bridgewater Treatise,' says, "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 proportion to the chambered part of the shell as the outer cell of the Nautilus Pompilius bears to the chambered interior of that shell. It often occupies more than half, add, in some eases, the whole circumference of the outer 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." It should be remembered that the specimen is apparently imperfect at the aperture. The siphon or tube of communication may be traced from d, where it opens into the last or outer chamber, along the edge of the section, e, f, g, h, i, to tho very nucleus of the shell. The a-aved transverse lines represent the partitions of the chambers.
The large preportion of the outer chamber is very strongly marked in specimens of A mastosites rostra! us, that have the aperture perfect or nearly so.
" Moreover," continues Dr. Buckland, "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" (loourrelei of the French), "giving to this part a strength which would apparently be needless to an internal shelL The presence of spines also in certain species (u in Ammonites ormolus, 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 boon useless, and perhaps noxious, in an internal position, and are without example in any internal structure with which we are acquainted."
Sir Henry do la 13eche has proved from the mineral condition of the outer chamber of Ammonites front the Liss at Lyme Regis, that the entire body was contained in it, these animals having been suddenly destroyed, and buried in the earthy sediment of which the Liu is composed, before their bodies had either undergone decay or been devoured by the then existing crustaceans.
Dr. Buckland very happily illustrates tho different arrangements by means of which a union of lightness and strength is secured to the shell, both from the external conformation and the mode in which the trans verse plates are disposed ; and as our limits will not allow us to enter minutely into the subject, wo must refer the reader to the 'Bridgewater Treatise' for the interesting details, which show that a more perfect instrument for affording universal resistance to external pressure—an instrument in which the greatest possible degree of lightness combined with the greatest strength was required--could scarcely be imagined ; and must confine ourselves to the doctor's summary :—"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 regu lated by a pipe passing through the whole series of tho chambers" (see the cut of Ammonites obtusua), "formed a hydraulic instrument of extraordinary 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, wo 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 trans verse plates that form the air-chambers supply also a continuous anocession of supports, extending their ramifications, with many mechanical advantages, beneath those portions of the shell which, being weakest, were most in need of them." who have written treatises on this interesting genus, or have illus. tiated The species of Ammonites are very numerous, and although the arrangement of Von Duch •is at present the best., it is probable that when more in known of the form of the aperture, it will servo as a leading character.
• Maris protnami Nautilos et Argonsutos, vulgo Cbrnoa Ammonia, in agro C.oburgico et victim reperiundos, deseripsit et &Uncivil, etc., IX 1. C. M. Rel. nerke. Coburg', 1818, 8ro.
t Veber die Ammoniten In den Ilteren Gehtrgs--Sehiehtcn. Gelesen in der Akademio der WIssensehaften, am 1 April, Me. (to. Reeuell do Munches de remarquables, par Leopold de Hoch. Berlin, 1831, folio.