AND NAUTILUS ZIC ZAC. 359 
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 com- 
parison of the lobes in N. Sypho (PI. 43, Fig. 2.), 
with a similar provision in the Nautilus Zic Zac 
(PI. 43, Figs. 3. 4.)=^ 
smaller extremity of the funnel a'. From c, to d, it lines the 
inside of the subjacent funnel a- ; and from d, continues down- 
wards to the termination of the funnel dfi, on the inside of e. 
Ate, 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 the air chambers. The area 
of this shelly tube is sutlicient, not only to have contained the 
distended siphuncle, but also to allow it to be surrounded with a 
volume of air, the elasticity of which would act in forcing back 
the pericardial fluid from the siphuncle, in the same manner as 
we have supposed the air to act within the chambers of the N. 
Pompilius. 
* On each side of the transverse plate in both these species 
there is an undulation, or sinus, producing lobes (PI. 43, Fig. 2. 
a^, a^, a^, 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 conjointly 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 in such a 
manner, that no portion of any lateral lobe is visible on the side 
here represented. At Fig. 2. a^, we see the projection of the 
lateral lobes, on each side of the convex internal surface of a 
transverse plate ; at a~ we see the interior of the same lobes, on 
the concave side of another transverse plate ; and at a^ the 
points of a third pair of lobes attached to the sides of the largest 
air-chamber that remains in this fragment. 
