272 COMPENSATIONS AND LINKS. 



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 PI. 43, Figs. 1, 2, 3, 4.,) 

 underpropping 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 com- 

 pensated 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 Am- 

 monite, 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 Ammo- 

 nite, compensating the disadvantages, that would other- 

 wise have followed from the marginal position of the 

 siphuncle in these two species, and the distances of their 

 transverse plates,* 



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 sec 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 frag- 

 ment. 



* In some of the most early forms of Ammonites which we find in the 

 Transition strata, c. 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, and of N. Zic zac; like tliem 

 also the margin was simple and destitute of fringed edges. The A. . 

 nodosus (PI. 40, Figs. 4 and 5.,) which is peculiar to the early Secondary 

 depositcs of the Muschelkalk, offers an example of an intermediate state, 



