354 NAUTILUS; NAUTILITE ; ORTHOCERATITE. 



by the elasticity of the air in the chambers, when it desires to 

 increase the bulk of the soft parts of its body. If such increase 

 and decrease in bulk could be effected without any change in 

 the weight of the whole mass, it would be caused (according to 

 well-known principles of Hydrostatics) to ascend or descend in 

 water ; the animal with its shell being altogether of so nearly 

 the same specific gravity with that fluid, that a very slight 

 difference in this respect may produce either effect. But this 

 theory, however ingenious, is inconsistent with the fact that, in 

 some of the fossil chambered shells of this group, which we only 

 know from their fossil remains, the siphuncle was evidently con- 

 tinued as a shelly tube throughout, and therefore could not have 

 been distended with fluid ; and even in the recent Nautilus, it 

 does not appear to have been possessed of sufficient elasticity, to 

 admit the action thus assigned to it. The use of the chambered 

 structure, and of the siphuncle, therefore, still remains unknown. 



896. The numbers of Fossil chambered shells formed on the 

 same plan, and therefore probably to be regarded as the remains 

 of Cephalopods of similar organisation, is very great. Thus we 

 find in almost all marine strata, from the oldest limestones and 

 sandstones of the Silurian system, down to those covering the 

 chalk, large numbers of shells, very nearly resembling the exist- 

 ing Nautilus, and therefore called Nautilites. Another fossil 

 genus, the Ortkoceratite, had a chambered shell, formed upon 

 the same plan with the Nautilus, but straight instead of being 

 spirally curved. In these two genera, as in some others allied 

 to them, and forming the family NAUTILID^E, the partitions or 

 septa between the chambers are smooth or simple ; that is, 

 although they are rather concave on the surface which looks 

 towards the outer chamber, and similarly convex on the other or 

 inner side, they have no inequality or irregularity of surface. 

 Moreover, in both instances, they are usually perforated by the 

 siphuncle nearly in their middle. 



897. In the spiral and straight shells which form the family 

 AMMONITHXE, on the contrary, the partitions are very sinuous 

 or wavy, sometimes even forming sharply-bent or zig-zag lines 

 (Fig. 549) ; and the siphuncle usually runs along the outer edge, 



