Page:Geology and Mineralogy considered with reference to Natural Theology, 1837, volume 1.djvu/278

 274 —Orthoceratite, Lituite, Baculite, Hamite, Scaphite, Turrilite, Nummulite, Belemnite.

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.

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.