SECTION V.
NAUTILUS SYPHO, AND NAUTILUS ZIC ZAC.
The name of Nautilus Sypho[1] has been applied to a very curious and beautiful chambered shell found in the Tertiary strata at Dax, near Bourdeaux; and that of Nautilus Zic Zac to a cognate shell from the London clay. (See Pl. 43, Figs. 1, 2, 3, 4.)
These fossil shells present certain deviations from the ordinary characters of the genus Nautilus, whereby they in some degree partake of the structure of an Ammonite.
These deviations involve a series of compensations and peculiar contrivances, in order to render the shell efficient in its double office of acting as a float, and also as a defence and chamber of residence to the animal by which it was constructed.
Some details of these contrivances, relating to the Nautilus Sypho will be found in the subjoined note.[2]
- ↑ This shell has been variously described by the names of Ammonites Atun, Nautilus Sypho, and N. Zonarius. (See M. de Basterot. Mem. Geol. de Bourdeaux.)
- ↑ The transverse plates, (Pl. 43, Fig. 1, a. a1. a2,) present a peculiarity of structure in the prolongation of the collar, or siphuncular aperture entirely across the area of the air chambers, so that the whole series of transverse plates are connected in one continuous spin] chain. This union is effected by the enlargement and elongation of the collar for the passage of the siphuncle into the form of along and broad funnel, the point of which b. its closely into the neck of the funnel next beneath it, c. whilst its inner margin, resting upon the arch of the subjacent whorl of the shell, transfers to this arch a portion of the external pressure upon the transverse plates, thereby adding to their strength.
As this structure renders it impossible for the flexible siphuncle to expand itself into the area of the air-chambers, as in other Nautili and in Ammonites, the diameter of each funnel is made large enough to allow
