moved and fed at the bottom of deep seas, and at other times rose and floated upon the surface.
The Nautili (see Pl. 31. Fig. 1. and Pl. 32. Figs. 1. 2.) constitute a natural genus of spiral discoidal shells divided internally into a series of chambers that are separated from each other by a transverse plates; these plates are perforated by a tube or siphon, passing through the transverse plates, either at their centre, or towards their internal margin. (Pl. 1. Fig. 31. Pl., 32. Fig. 2. and Bl. 33.)
The external open chamber is, very large, and forms the receptacle of the body of the animal. The internal close chambers are void, and have no communication with the outer chamber, excepting for the passage of a membranous tube, which descends through an aperture in each plate to the innermost extremity of the shell, (Pl. 31, y. y. a. b. c. d. e. and Pl. 32, a. b. d. e. f.) These air chambers are destined to counterbalance the weight of the increasing body and shell of the animal, and thereby to render the whole so nearly of the weight of water, that the difference arising from the membrane of the siphuncle being either empty, or filled with a fluid, may cause the mass to swim or sink.[1]
- ↑ The siphuncle represented in Pl. 31, Fig. 1, illustrates the structure and uses of that organ; in the smallest whorls, from d. inwards, it is enclosed by a thin calcareous covering, or sheathr, of so soft a nature as to be readily scraped off by the point of a quill; this sheath may admit of expansion or contraction, together with the membranous tube enclosed within it. In the fossil Nautili, a similar calcareous sheath is often preserved, as in Pl. 32, Figs. 2, 3, and Pl. 33, and forms a connected series of tubes of carbonate of lime, closely fitted to the collar of each transverse plate. In four chambers of the recent shell (Pl. 31, Fig. 1, a. b. c. d.) this sheath is partially removed from the desiccated membranous pipe within it, which has assumed the condition of a black elastic substance, resembling the black continuous siphuncular pipe that is frequently preserved in a carbonaceous state in fossil Ammonites.
At that part of each transverse plate, which is perforated for the passage of the siphuncle, (Pl. 31, Fig. 1, y. y.,) a portion of its shelly matter projects inwards to about one-fourth of the distance across each chamber, and forms a collar, around the membranous pipe, thus directing its pas-
