382 M. Von Buch on the characteristic Fossils 
the Exogyre. A few words explaining the general character of 
these forms may not prove unacceptable. 
1. Toe AMMONID&. 
It is well known, that the Ammonites, still so remarkably 
abundant in the lowest cretaceous beds, or the Néocomien, ra- 
pidly decrease in the higher strata; so much so, that even in the 
upper chalk they no longer occur, and that every trace of them 
has since vanished from the surface of the earth. There is how- 
ever something very remarkable in the manner of this disappear- 
ance. The greater number of the Ammonites seem to suffer 
- from the disease which at length expels them from creation. 
The whorls in many no longer lie exactly in one plane, but the 
one side projects somewhat forwards and draws the other after 
it. Soon after they even lose the power to attach themselves 
closely to the previous whorl; and these are now unconnected 
(F. A. Romer, Kreide, p. 135), when the Crioceras, which is only 
found in the chalk, is formed. The formation of such unattached 
whorls soon exceeds the power of the animal to bend its body, 
and thus to contract itself into such protecting limits,—it is 
compelled to extend itself lengthwise, and the singular forms of 
the Toxoceras, the Ancyloceras, the Ptychoceras, finally those of 
the Hamites, and of the Baculites, straight like a staff and directed 
perpendicularly upwards, are produced, and are the last attempts 
of the animal to maintain its existence. Subsequently nothing 
appears in nature which can remind us of this kind of Cepha- 
lopod. All these forms therefore, diverging from the perfect Am- 
monite, mark in the most decided manner one or other portion 
of the cretaceous formation,—they are leading forms, which, where 
they occur, exclude the supposition of any other formation. 
It is truly remarkable, that in the same manner as the Ammo- 
nide vanish from the world, in the same manner exactly do the 
Nautilide make their appearance in the oldest strata. The en- 
tirely straight Orthoceratites are the most ancient of all known 
Cephalopods ; they endeavour, at least when young, to attach 
themselves to the previous whorl, an attempt which, however, 
must be given up in its further growth, when the animal can only 
increase lengthwise ;—the Lituite is produced ;—the Clymenie 
succeed in attaching themselves throughout to the previous 
whorl, in a continuous spiral entirely situated in one plane ; the 
Nautilus finally—of which a weak remnant, the Nautilus pompi- 
dius, has continued to the present time—surrounds all the pre- 
vious whorls with the last one, and thus withdraws itself more 
perfectly from the attacks of its enemies. The Ammonite va- 
nishes through a series of forms intermediate between it and the 
