CLASS I. CEPHALOPODA: ORDER 2. TETRABRANCHIATA. 503 



SECTION OF A NAUTILUS SHOWING ITS INTERIOR. 



THE NAITTILID^. 



Of this there are many genera and species, all fossil but the following : 



Genus NAUTILUS : Nautilus. — This includes the Pearly Nautilus, N. Pompilius : it has 



four hranchife, 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 poulpc, 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 AMMONITID^. 



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 Corrm Ammonis was given them from a fancied 

 resemblance to the horns with which the head of Jupiter Amnion 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 w^orms 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. 



