Orthoceras (straight shell), [Lign. 155].—The shells of this genus may be described as Nautili uncoiled and extended in a straight line. They are straight, elongated, chambered shells, with smooth and gently undulated septa, which are concave towards the opening or upper part, and have the siphuncle either central, or not far removed from the centre. The Orthoceratites more especially belong to the ancient Secondary strata. They first appear in the Silurian, and abound in the Devonian and Carboniferous. They vary in size from a few inches to several feet in length, and eight or nine inches in diameter; and in form, from a slender elongated cone, to a short, massy, almost spherical figure, with a contracted orifice. Some examples have been noticed with upwards of sixty cells. Mr. Sowerby figures and describes O. giganteum (Min. Conch. tab. 246), from Scotland, as exceeding seven or eight feet in length; and I discovered on the beach at Brighton, where it had probably been brought by some vessel, among ballast, a fine fragment of the same species, indicating as great a magnitude. Several species are figured, [Lign. 155], to show the structure and appearance of these fossils. The casts of the separate cells are often found. The section, fig. 3, from the red marble of Devonshire, beautifully displays the situation of the siphuncle, and the lines of the septa. The shelly siphuncle, which is moniliform (bead-like), or dilated at each chamber, is replaced by white spar; and the membranous internal tube is filled with a dark substance, probably molluskite.
Lign. 155. Orthoceratites.
| Fig. | 1.— | Orthoceras striatum. (Min. Conch.) Devonian. |
| 2.— | Orthoceras conicum. Whitby. Carboniferous. | |
| 3.— | Vertical section of an Orthoceras, showing the central siphon,and the chambers. Devonshire. | |
| 4.— | Orthoceras laterale. Carb. (Min. Conch.) | |
| 5.— | Orthoceras gregarium. (Munch. Sil. Syst.) a. One of the septa. b. A portion covered at the upper part by the shell. c. The lower part of the same specimen, displaying the septa. |
There are some species in which the internal tube, as well as the external, is calcareous, and the two are connected at regular intervals, by radiating, hollow processes. These Orthoceratites have been principally obtained from the Silurian limestones, at Lake Huron; they also occur in Ireland. Mr. Stokes, who first investigated their structure, has arranged them in a distinct genus, with the name of Actinoceras (radiated-horn).[413]
[413] See Geol. Trans, second series, vol. v. p. 708.
Slabs of reddish Devonian limestone, containing Orthoceratites, may be seen in some of the pavements at Hampton Court, and in Chelsea College, which when wet present excellent sections of the enclosed shells.
