NATURAL HISTORY OF THE PRIMEVAL WORLD. 



Every animal was provided, like the modern Sepia* 

 with an ink-bag nearly a foot in length. Hence we 

 may conclude that they had an internal shell ; the ink- 

 bag being a defensive provision entirely confined to 

 naked Cephalopods. 



From the weight of their internal shell we may sup- 

 pose that they usually maintained a vertical position ; 



lleleimiite restored. 



and as their chambered portion was supplied with a 

 locomotor tube, or siphuncle, like the Nautilus, they 

 probably rose and sunk in the water with the utmost 

 facility. 



These fossils are popularly known as Arrow-heads, 

 Thunderstones, and Fingerstones. The species are 

 nearly 100 in number. The reader should remember 

 that they are not the animals themselves, but the 

 internal organisms of those animals ; naked cephalo- 

 pods, which protected themselves by clouding the water 

 with discharges of inky fluid, just as Homer's gods and 

 goddesses, when threatened by any danger, surrounded 

 themselves with an impenetrable cloud or mist. 



The numerous family of the Ammonites now demands 

 our notice. Their essential parts, it will be seen, were 

 so similar in principle to those of the Nautili shells, 

 that we cannot doubt they answered a like purpose in 

 the economy of the extinct species of cephalopodous 

 molluscs from which these Ammonites have been 

 derived. 



They received their name from the resemblance of 

 their beautiful shell to the ram's-horn decorations which 

 symbolically enriched the front of the Temple of Jupi- 

 ter Ammon, and the bas-reliefs and statues of the Pagan 

 deity. Shells are all that remain of them. The living 

 organism has long since disappeared. 



Like the Belemnite, and like the Nautilus, the Am- 

 monite consisted of three essential parts- 

 First, an external shell, usually of a fiat, disc-like 

 form, having its surface strengthened and ornamented 

 with ribs. 



Second, a series of internal air-chambers formed by 

 transverse plates intersecting the inner portion of the 

 shell. 



Third, a siphuncle, siphon, or locomotor tube, which 

 began at the bottom of the outer chamber, and per- 



* Professor Owen has demonstrated that the animal of which 

 the Belemnites was the internal bone, was, in reality, a di- 

 branchiate eight-armed cuttle-fish, like the modern genus Ony- 

 c/ioteuthis. 



forated the entire series of air-chambers to the inner- 

 most extremity of the shell. 



The uses of the shell of the Ammonites an exter- 

 nal, not internal shell, let it be remembered have 

 been frequently discussed. They have been demon- 

 strated by Dr. Buckland with a truth and clearness 



Ammonites restrains. 



unsurpassed by any later writer, and our description 

 will closely follow his very graphic and interesting 

 account. 



As the shell served the twofold office of affording pro- 

 tection to the internal organism, and enabling it to float, 

 it was necessary that it should be thin, or its weight 

 would prevent it from rising to the surface. But it was 

 not less necessary that it should be strong enough to 

 resist the pressure of the mass of waters at the bottom, 

 or in the mid-depths of the sea. Accordingly, we find 

 that its Creator has admirably adapted it for its double 

 function by so disposing its materials as to combine 

 strength and firmness with lightness and buoyancy. 



Let it be noticed, in the first place, that the entire 

 shell is one continued arch and no form, as any archi- 

 tect will tell you, is better fitted to resist superincum- 

 bent pressure is one continued arch, we say, coiled 

 spirally around itself in such a manner, that the base 

 of the outer whorls rests upon the corner of the inner 

 whorls, and the keel or back is thus calculated to resist 

 weight, on the same principle that the shell of a com- 

 mon hen's egg will endure considerable force, if that 

 force be applied in the direction of its longitudinal 

 diameter. 



Let it be noticed, secondly, that, in addition to this 

 general arch-like form, the shell is further strengthened 

 by the insertion of ribs, or transverse arches, which 

 give to many of the species their characteristic feature, 

 and in all produce that peculiar beauty which, for every 

 artist's eye, invariably distinguishes the symmetrical 

 repetition of a series of spiral curves. 



From the disposition of these ribs over the surface 

 of the external shell, mechanical advantages are obtained 



