382 M. Von Buch on the characteristic Fossils 



the Exogyra. A few words explaining the general character of 

 these forms may not prove unacceptable. 



1. The Ammonid^e. 



It is well known, that the Ammonites, still so remarkably 

 abundant in the lowest cretaceous beds, or the Neocomien, 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- 

 niola vanish from the world, in the same manner exactly do the 

 Nautilidce 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 Clymenica 

 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- 

 lius, 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 



