—Orthoceratite, Lituite, Baculite, Hamite, Scaphite, Turrilite, Nummulite, Belemnite.[1]
Orthoceratite, Pl. 44, Fig. 4.
The Orthoceratites (so called from their usual form,—that of a straight horn) began their existence at the same early period with the Nautili, in the seas which deposited the Transition strata; and are so nearly allied to them in structure, that we may conclude they performed a similar function as floats of Cephalopodous Mollusks. This genus contains many species, which abound in the strata of the Transition series, and is one of those which, having been called into existence amongst the earliest inhabitants of our planet, was at an early period also consigned to almost total destructions.[2]
An Orthoceratite (see Pl. 44, Fig. 4) is, like the Nautilus,
a multilocular shell, having its chambers separated by transverse
plates, concave externally, and internally convex; and
pierced, either at the centre or towards the margin, by a
Siphuncle, (a.) This pipe varies in size, more than that of
any other multilocular shell, viz. from one-tenth to one-half
of the diameter of the shell; and often assumes a tumid
form, which would admit of the distension of a membranous
Peron having been lost, is in some degree removed by Captain King's discovery of another of these shells, attached to a fragment of the mantle of an animal of unknown species resembling a Sepia, which I have seen in the possession of Mr. Owen, at the Royal College of Surgeons, London.
- ↑ In the genus Lituite, Orthoceratite, and Belemnite, Pl. 44, f. 3, 4, 17, the simple curvature of the transverse plates resembles the character of the Nautilus. In the Baculite, Hamite, Scaphite, and Turrilite, Pl. 44, Fig. 5, 8, 12, 13, 14, 15, the sinuous foldings and foliated edges of the transverse plates resemble those of the Ammonites.
- ↑ See D'Orbigny's Tableau Méthodique des Céphalopodes.
There are, I believe, only two exceptions yet known to the general fact, that the genus Orthoceratite became extinct before the deposition of the Secondary strata had commenced. The most recent rocks in which they have been noticed, are a small and problematical species in
