358 CONTRIVANCES IN NAUTILUS SYPHO 
to the Nautilus Sypho will be found in the sub- 
joined note.* 
As the place of the siphon in this species is 
upon the internal margin of the transverse plates 
(PI. 43, Fig. 2, b\b%b\) 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 
* The transverse plates, (PI. 43, Fig 1, a. a‘. a^.), 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 spiral chain. This union is effected by the enlarge- 
ment and elongation of the collar for the passage of the 
siphuncle into the form of a long and broad funnel, the point of 
which b. fits closely into the neck of the funnel next beneath it, 
c. whilst its inner margin, resting npon 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 siphun- 
cle 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 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 siphun- 
cle is contracted, as the siphunclesof Ammonites and Nautili are 
contracted at their passage throtigh 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, 
(PI. 43, Fig. l,b. c. d.), analogous to that of the N. Porapilius, 
(PI. 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 
