THE CEPHALOPODS. 255 
spiral, so much so, that the last turn envelops the others; and 
what particularly characterises it is, that it is divided into chambers 
by concave walls, which it would seem the creature builds up 
behind it. These chambers are all connected by a tube which 
passes through their centre, called the s¢phuncle. The animal lives 
in the upper chamber of the shell, and by regulating the quantity 
of air or water the walled-up compartments contain, nicely 
adjusts the weight of the shell. The structure is so delicate, 
that if it struck against the ground it would be broken; and as 
the creature when it moves along the bottom does so by using its 
arms, it cannot drag its shell behind it, it therefore causes it to 
have the same specific gravity as the water; as it thus swims 
above it, it in no way impedes the progress of the cephalopod. 
The nautilus was represented in the seas of ages long past 
by a cephalopod now found in a fossil state, and called after its 
likeness to the horns of the image of Jupiter Ammon—the 
Ammontte. It differs from the nautilus in having the siphuncle 
situated at the margin of the shell, and not passing through the 
centre of the chambers; moreover, the last chamber is not so 
disproportionately large. 
These ammonites have the appearance of snakes coiled up 
and deprived of their heads; and the popular idea on the coast 
of Yorkshire, where the fossils are found in abundance, is that 
when St. Patrick banished the vermin from Ireland, the snakes 
in the agonies of death coiled themselves up, and are found 
entombed in the English clay, witnesses to the power of the 
saint. 
