As the place of the syphon in 'this species is upon the internal margin of the transverse plates (Pl. 43; Fig. 2 b1, b2, b3,) it had less power than the more central siphuncle of the Nautilus to attach the mantle of the animal to the bottom of the outer chamber. For this defect we find a compensation, resembling that which Von Buch considers to have been afforded by the lobes of Ammonites to the inhabitants of those shells. This compensation will be illustrated by a comparison of the lobes in N. Sypho (Pl. 43, Fig. 2.,) with a similar provision in the Nautilus Zic Zac (Pl. 43, Figs. 3, 4)[1]
- ↑ On each side of the transverse plate in both these species these is an undulation, or sinus, producing lobes (Pl. 43, Fig. 2. a1, a2, a3, Fig. 3. a. and Fig. 4. a. b.) There is also a deep backward curvature of the two ventral lobes, Fig. 4. c. c. All these lobes may have acted can jointly with the siphuncle, to give firm attachment to the mantle of the animal at the bottom of the outer chamber. The shell Fig. 1. is, broken
space within it for the distension of the siphuncle, by a sufficient quantity of fluid to cause the animal to sink.
At each articulation of the funnels, the diameter of the siphuncle is contracted, as the siphuncles of Ammonites and Nautili are contracted at their passage through the collars of their transverse plates. Another point in the organization of the siphuncle is illustrated by this shell, namely, the existence of a soft calcareous sheath, (Pl. 43, Fig. 1, b. c. d.) analogous to that of the N. Pompilius, (Pl. 31, Fig, 1, a. b. c. d.) between each shelly funnel and the membranous pipe or siphuncle enclosed within it. At Pl. 43, Fig. 1, b, we have a section of this sheath folding round the smaller extremity of the funnel a'. From e, to d, it lines the inside of the subjacent funnel a3; and from d, continues downwards to the termination of the funnel a3, on the inside of e. At e, and f; we see the upper termination of two perfect sheaths, similar to that of which a section is represented at b. c. d. This sheath, from its insertion between the point of the upper siphon and mouth of the lower one, (Fig. 1, c.,) must have acted as a collar, intercepting all communication between the interior of the shelly siphuncular tube and their chambers. The area of this shelly tube is sufficient, not only to have contained the distended siphuncle, but also to allow it tube surrounded with a volume of air, the elasticity of which would act in forcing back the pericardia fluid from the siphuncle, in the same manner as we have supposed the air to act within the chambers of the N. Pompilius.
