18 CEPHALOPODA. NAUTILUS. 
Genus XII.— NAUTILUS. — Linnaeus. 
Generic Character. — Shell suborbicular, multilocular, 
convolute, with contiguous volutions, and simple partitions ; 
septa transverse, and externally concave; perforated in the 
disk ; margins entire ; aperture ample. 
Nautilus striatus. Plate II. fig. 26. 
This genus contains numerous fossil species, and there are two existing ones 
in tropical seas. The fossil Nautili afford an excellent example of the limita- 
tion of certain species to particular geological formations. The N. multicari- 
natus, is confined to strata of the Transition series; the WN. bidorsatus, to the 
Muschelkalk ; the NW. obesus and lineatus, to the Oolite series; WN. elegans and 
undulatus to the Chalks; and the divisions of the Tertiary formations contain 
also species of Nautili peculiar to themselves. 
On a careful review of all the fossil Nautili, and a comparison with the only 
two existing species of this genus, it will be found, that they have retained, 
through strata of all ages, their primitive simplicity of structure, down to the 
present time. They were among the earliest inhabitants of the ancient seas of 
our globe, and have preserved their generie distinctions through all the revo- 
lutions to which the earth’s surface has ever been subjected. While its kindred 
multilocular genera have become entirely extinet. This is the more remarkable, 
as there are not exceeding sixty species of Nautili, while it will be seen upwards 
of 270 species of fossil! Ammonites have been ascertained, not a single one of 
which has survived the wreck of the former world, nor does a type of the 
genus now exist. 
We must explain the functions of this genus of Mollusks, by a reference to 
one of the only existing types, the Nautilus Pompilius. Plate I. fig. 3. 
The only organ connecting the air chambers with the body of the animal, is 
the siphuncle, which penetrates the aperture, and the short projecting tube 
{c) running through each successive partition, till it terminates in the inner- 
most chamber of the shell. By means of this mechanism, the animal can 
either augment or diminish its specific gravity, on the same principle as fishes 
distend or collapse their air bladders ; so that when the siphon is filled with 
water, the entire body is rendered specifically heavier than the surrounding 
fluid, and consequently sinks. When the water is expelled, it naturally rises 
to the surface and floats. 
The Rhyncholites, or Beak-stones, found plentifully im the Oolite of Stones- 
field, the lias at Bath and Lyme Regis, and in the Muschelkalk at Luneville, 
are the fossil beaks of Nautili and Ammonites. See plate I. fig. 5.* 
The animal resides in the outer chamber only, and when too large for it, a 
new partition is formed of dimensions sufficiently ample to accommodate the 
inhabitant, and thus its former abode is converted into an additional air cham- 
ber. We have represented a cast of an airchamber. Plate I. fig. 11. 
There is a beautiful contrivance in the shells of this genus, for increasing 
their strength, and enabling them to withstand the pressure of the fluid by 
which they are surrounded ; namely, the eaternal transverse lines of growth 
or ribs, (as in plate II. fig. 26,) having a different curvature from the inter- 
nal transverse plates or partitions; the internal partitions being eonvex 
inwards, while the outer ribs of the shell are convex owtwards ; these ribs 
* For a minute detail of the construction of the shell of this animal, see the admirable 
** Memoirs on Nautilus Pompilius,’”’ by Professor Owen. 
