360 COMPENSATIONS AND LINKS. 
A still more important use of the lobes formed 
by the transverse plates both of IS. 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.), underpropping theirflattest 
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 neces- 
sary, may be found in the breadth of the inter- 
vals 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 posi- 
tion of the siphuncle in these two species, and 
the distances of their transverse plates.* 
* 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, PI. 40, Figs. 1, 2, and 3,) the lobes were few, and 
nearly of the same form as the single lobe of the Nautilus Sypho, 
