The shell of the Ammonite is generally thinner and more delicate than that of the Nautilus. However thin these shells may be, they are possessed of great strength of structure. Not only is the shell one continuous arch, but it is moreover provided with transverse arches or corrugated ribs, which, like the flutings on metal pencil-cases, and corrugated sheet-iron, greatly strengthen the thin material. These ribs are further subdivided, so as to multiply supports as the convexity of the shell enlarges, in a manner somewhat similar to the groin-work of vaulted roofs. The spines, tubercles, and bosses, which often arise from the ribs, are so many additions to the strength of the latter. The sides of the shell are also supported by the transverse plates forming the septa, or divisions of the chambers, and, as these plates are very sinuous where they meet the sides of the shell, they distribute their support over a considerable portion of the surface.[416] These complicated edges of the septa form the delicate and intricate lines of foliation (sutures) seen on the casts of Ammonites ([Lign. 156], fig. 2), Baculites ([Lign. 161], fig. 3), Hamites, &c. In some species the shell is thick and dense, as in the Ammonites of the Kimmeridge Clay near Aylesbury, in which one species (Ammonites biplex, Min. Conch. pl. ccxciii.) in particular abounds, and is very generally invested with the shell, as perfect as if recent. The same Ammonite occurs in the Portland limestone above, in the state of casts, without any vestige of the shell. In some strata the shell is replaced by calcareous spar; in others by silex or flint.[417] In the pyritous clays and shales of the Lias, the shell and all its delicate internal mechanism are coated with or replaced by brilliant sulphuret of iron, forming the most exquisite natural electrotype imaginable. Polished vertical sections of these fossils often exhibit the inner cells filled with transparent white calcareous spar; sometimes with groups of crystals of sulphate of lime. The Ammonites of the Galt, and of the Kimmeridge Clay, are also frequently imbued with the same mineral.
[416] See Bd. i. p. 339, &c.
[417] See an admirable figure of a chalcedonic specimen, exhibiting the foliated septa of an Ammonite, Bd. pl. xli.
There are about two hundred identified species of Ammonite in the British strata, ranging through all the secondary formations; they have not been found in the Tertiary deposits. They vary in size from half an inch to four feet in diameter.[418]
[418] I have seen imprints of the large Chalk Ammonite, A. peramplus (Min. Conch. pl. ccclvii.), on the shore off Rottingdean, and Beachy Head, which indicated even larger proportions.
Certain species are restricted to particular formations, and are therefore oftentimes of essential aid in determining the relations of a deposit; for example, the Galt contains several species not found in the upper division of the Chalk; and in the Chalk-maid are species that have not been discovered in other strata. Certain Ammonites of the Lias are peculiar to that formation (as A. Walcottii, [Lign. 127], fig. 7, p. 397; and A. communis, [Lign. 157]). Ammonites of species allied to those of our Middle and Lower Oolites have been discovered in strata in the Himalaya mountains, several thousand feet above the level of the sea.
Lign. 159. Goniatites. nat.
Carb. Syst.