A still more important use of the lobes formed by the transverse plates both of the N. Sypho and N. Zic Zac, may be found in the strength which they impart to the sides of the external shell (see Pl. 43, Figs. 1, 2, 3, 4.,) under propping their flattest and weakest part, so as to resist pressure more effectually than if the transverse plates had been curved simply, as in N. Pompilius. One cause which rendered some such compensation necessary, may be found in the breadth of the intervals between each transverse plate; the weakness resulting from this distance, being compensated by the introduction of a single lobe, acting on the same principle as the more numerous and complex lobes in the genus Ammonite.
The N. Sypho and N. Zic Zac seem, therefore, to form Links between the two great genera of Nautilus and Ammonite, in which an intermediate system of mechanical contrivances is borrowed, as it were, from the mechanics of the Ammonite, and applied to the Nautilus. The adoption of lobes, analogous to the lobes of the Ammonite, compensating the disadvantages, that would otherwise have followed from the marginal position of the siphuncle in these two species, and the distances of their transverse plates.[1]
- ↑ In some of the most early forms of Ammonites which we find in the Transition strata, e. g. A. Henslowi, A. Striatus, and A. Sphericus, (Pl. 40, Figs. 1, 2, and 3,) the lobes were few, and nearly of the same form as the single lobe of the Nautilus Sypho, and of N. Zic zac; like them also the margin was simple and destitute of fringed edges. The A. nodosus (Pl. 40, Figs. 4 and 5.,) which is peculiar to the early Secondary deposites of the Muschelkalk, others an example of an intermediate state,
in such a manner, that no portion of any lateral lobe is visible on the side here represented. At Fig. 2. a1, we see the projection of the lateral lobes, on each side of the convex internal surface of a transverse plate; at a2 we see the interior of the same lobes, on the concave side of another transverse plate; and at a3 the points of a third pair of lobes attached to the sides of the largest air-chamber that remains in this fragment.
