CLASS I. CEPHALOPODA: ORDER 2. TETRABRANCHIATA. 603 
SECTION OF A NAUTILUS SHOWING ITS INTEEIOR. 
THE l^AUTILIDJE. 
Of this there are many genera and species, all fossil but the following : 
Genus JSTAUTILUS : Nautilus. — This includes the Pearly Nautilus, JSf. Pompilius : it has 
four branchia3, and an external shell of a beautiful 
pearly texture and color, to which the animal is per- 
manently attached. This shell consists of a series of 
chambers, pierced through the middle by a tube or 
siphuncle, which extends to the remotest cell. The 
body and limbs of the -nautilus resemble in some re- 
spects those of the poulpe, and are contained in the 
outer receptacle of the shell ; it maintains a vital con- 
nection with the inner chambers by means of a mem- 
braneous tube which lines the siphuncle, passing through 
the internal vacancies, these being merely air-cham- 
bers to give the whole the necessary specific gravity, 
so as to float or sink in the water at the animal's 
pleasure. It lives in deep water,* and feeds at the 
bottom of the sea, where it crawls about, carrying its 
house on its back like a snail. It feeds on small Crus- 
tacea and other sea-animals. It is sometimes seen 
floating on the surface of the water, the head and ten- 
tacles spread out, and the shell riding like a boat. This species, and two or three others closely 
resembling it, are found in the Asiatic seas. 
The shells of a hundred extinct species, found in Europe, America, and Asia, are known to 
the geologists. 
THE AMMOi^ITID^. 
These are all fossil species, but they are so remarkable as to demand notice. They were of 
various forms, but they were of carnivorous habits, resembling the 
sepias, and, like the nautili, living in chambered shells. These shells 
are found in various situations in vast numbers, from the size of a pin's 
head to the diameter of the largest cart-wheel. No less than five hun- 
dred species are known. They appear to have attracted attention from 
a very early date, and to have been the source of many curious specu- 
lations. The name of Cornu Animonis was given them from a fancied 
resemblance to the horns with which the head of Jupiter Ammon was 
sculptured; hence they are called Ammonites. In the earlier times 
their origin was variously accounted for. Some thought them petrifactions of real rams' horns, 
taking the name above mentioned in a strict sense ; others thought they were the curled tails of 
certain animals ; some took them for petrified marine worms rolled up ; others saw in them coiled 
serpents, whence they were called snake-stones. The legends of the saints invested them with a 
sacred interest : 
" Of thousand snakes, each one 
Was changed into a coil of stone 
When holy Hilda pray'd." 
And the prayer, we are told, was not only followed by petrifaction, but by decapitation. There 
is a similar tradition of St. Keyna, who, when she found herself in a wood at Keynsham, between 
AMMONITE. 
* By deep water naturalists and dredgers seldom mean more than twenty-five fathoms, a comparatively small 
depth, only found near coasts and islands. At one hundred fathoms the pressure exceeds two hundred and sixty- 
five pounds to the square inch. Empty bottles, securely corked, and sunk with weights beyond one hundred fath- 
oms, are always crushed. If filled with liquid, the cork is driven in, and the liquid replaced by salt water; and in 
drawing the bottle up again, the cork is returned to the neck of the bottle, generally in a reversed position. 
