Lign. 156. Ammonites from the Cretaceous Formation.
| Fig. | 1.— | Ammonites varians. Chalk-marl. Hamsey. |
| 2.— | Ammonites Dufrenoyi. | |
| 2a.— | Shows the keel and septum of the same. | |
| 3.— | Ammonites lautus. Galt. Folkstone. | |
| 3a.— | Keel and septum of the same. |
Ammonitidæ.—The Ammonites, or Cornua Ammonis (so called from a supposed resemblance to the horns engraven on the heads of Jupiter Ammon), are among the most common and well-known fossils of the British secondary strata. In some districts, as in Yorkshire and Somersetshire, where the Ammonites abundantly prevail, they were noticed in very remote times. Local legends, ascribing their origin to swarms of snakes turned into stone by the prayers of some patron saint, are still extant, and are perpetuated by the name of snake-stones, by which these fossils are provincially known. The Lias, near Whitby, in Yorkshire, contains immense numbers of two or three species, one of which (Am. bifrons) is figured in [Lign. 127], fig. 7, and another in [Lign. 157].
Lign. 157. Ammonites communis.
Lias. Whitby.
The shells comprehended in this family are either spiral, involute, arched, or straight; their septa are deeply lobed, and have the margins foliated. The siphuncle is dorsal, as shown by the notch in the cast, [Lign. 156], fig. 3a. Several hundred species have been described; they are divided into genera which are characterized by essential modifications in the direction of the spire, and the inflections of the septa. Thus, in the Ammonites, [Lign. 156], the spire is involute, and all the turns contiguous; in Crioceras (curved-horn), [Lign. 160], fig. 2, evolute; in Scaphites, incurved at both extremities, [Lign. 162]; Hamites, bent like a siphon, or hook, [Lign. 161], fig. 1; Turrilites, spiral, round a vertical axis, [Lign. 163]; and in Baculites, straight, [Lign. 161], fig. 2. New genera are continually being added, to embrace modifications of structure which appear to be too important for specific distinctions. I will endeavour to render this arrangement more clear to the student by the following definitions.
A straight tube, or horn, of an elongated conical figure, tapering to a point, and having its cavity divided by transverse partitions, which septa are not straight, but undulated, and their edges, which fit into the walls of the tube, deeply wrinkled, and the whole series pierced by a pipe running along near the outer margin, would be the model of the shell termed Baculites, [Lign. 161], fig. 2, (Bd. pl. xliv. fig. 5;) which may be regarded as a straight Ammonite. A similar shell, gently arched or curved, would be a Toxoceras, [Lign. 160], fig. 1; the same tube, bent upon itself, like a siphon, into unequal limbs, not contiguous, a Hamites, [Lign. 161] (Bd. pl. xliv. fig. 10); bent and approximate, or anchylosed in a straight line, Ptychoceras, [Lign. 161], fig. 4; partially convoluted, the whorls contiguous, and the free end recurved, Scaphites, [Lign. 162]; the same form, but the spire not contiguous, Ancyloceras, [Lign. 160], fig. 3; spirally twisted around an axis, Turrilites, [Lign. 163] (Bd. pl. xliv. fig. 14); coiled, but the turns not touching each other, Crioceras, [Lign. 160], fig. 2; lastly, coiled up in the form of a disk, all the turns being contiguous, Ammonites.
AMMONITE.