2. The Orthoceratidæ ([Lign. 155]): in these the siphuncle is complicated in its structure; it is central or lateral; the septa are smooth. (Woodward’s Man. Moll. p. 87.)
3. The Ammonitidæ (Bd. pl. xxxv-xlii.): in these the septa are more or less waved, and their margins foliated or crenated, that is, indented; and the siphuncle is situated at or near the outer margin.
In the Nautilus, the shell is convoluted on the same plane, in spiral whorls, all of which are contiguous, and the siphuncle is central.
The British strata contain about sixty species of Nautili. The Tertiary formations have yielded five or six; the Cretaceous a like number; the Lias and Oolite ten or eleven; the Carboniferous about thirty species; and the Devonian two species. In the London Clay a large and beautiful species is abundant (Nautilus imperialis. Min. Conch. tab. i.), having the shell very commonly entire; but the outer opaque coat frequently flakes off, and exposes the pearly or nacreous internal layer. The septa generally retain their original nacreous structure, and the cells are either occupied by clay or marl, or are partially filled or lined with calcareous spar, brilliant pyrites, or other mineral matter. These Nautili are often found constituting the nuclei of the septaria, or clay nodules, with which this deposit abounds.[410] The small species, N. centralis (Ly. fig. 179), and Nautilus (Aturia) ziczac (Wond. p. 247), occur in the same strata. The London Clay of the Isle of Sheppey and of the coasts of Hants and Sussex is productive of these fossils.
[410] Three other well-marked species are figured and described by Mr. Edwards (Monog. Pal. Soc.) from the English eocene strata: viz. N. Sowerbyi, N. urbanus, and N. regalis.
Lign. 150. Fossil Nautili. Chalk marl.
| Fig. | 1.— | Beak of a Nautilus, (Rhyncolite.) Back view. |
| 1a.— | Anterior view of the same. | |
| 1b.— | Profile of the same. | |
| 2.— | Vertical section of Nautilus pseudo-elegans. Hamsey. a. The siphuncle. | |
| 3.— | Front view of N. Deslongchampsii. (M. D’Orbigny.) Hamsey. | |
| 3a.— | Lateral view of the same. |