320 



THE GEOLOGIST. 



the fragile nature of the shell, greater guarantee for their 

 geological distribution than those shells infinitely more liable 

 to be covered up in the strata. 



§3. Relation of zoological characters of Ammonites^ with the 

 different geological epochs at which they have existed. 



I have stated that all the species of Ammonites, of each 

 geological group, may be considered characteristic. It re- 

 mains at present to inquire whether, among these species 

 there are groups of forms peculiar to these -geological periods 

 and sufficient to distinguish more truly than isolated species, 

 the changes which have taken place at different epochs, thus 

 furnishing to Geologists fresh means of distinguishing by 

 zoological characters these epochs from each other. 



If, before entering, into the general consideration of the 

 external and internal forms of Ammonites, to determine the 

 positive characters of groups, we seek, in the purely external 

 appendages, a more general fades, which may serve to dis- 

 tinguish the general form of the Ammonites of each geolo- 

 gical group, we arrive at the following results : 



1st. Amongst the Ammonites of the Neocomien, an ex- 

 ternal character, which strikes us at once, and belongs 

 especially to this formation is the presence, in thirty Ammo- 

 nites out of sixty-five, of ridges or transverse ribs, which 

 represent, at certain distances on each whorl, the periods of 

 rest in the deposition of the shell. This remarkable cha- 

 racter is especially observable in the following Ammonites : 

 A. Belus, A, Flexisulcatus. 



Calypso, Guettardi. 



Cassida, Honnoratianus, 



Cesticulatus. Inceqicalicostatus, 



Difficilis. Incertus. 



Dispar. Intermedins, 



Duvalianus. Juilleti, 



Emerici, Lepidus, 



