STABILITY OF THE GENERA. 53 



III. Elements of the Shell. 



The particular study of each of the elements of the shell of 

 the cephalopods, of which we have presented a resume, shows 

 that none of them have followed a gradual variation in any one 

 direction. On the contrary, we have observed the stability of 

 these elements, notwithstanding their specific and temporary 

 variations. Those facts are manifested above all in the genera 

 of which the duration has been the longest, as Orthoceras and 

 Nautilus. 



IV. Stahilitij of Iliv Genera. 



1. Orthoceras takes the second rank, by its longevity, among 

 the generic types, but the first by its richness in species, since 

 it furnished alone about half those of the Palaeozoic cephal- 

 pods. It traverses every age, commencing with the origin of 

 the second fauna, to the extinction of the Triassic fauna. Its 

 species, very diverse in every horizon, preserve nevertheless 

 their typical features very plainly, in about 1.146 forms 

 enumerated in our studies. 



Among the proofs of the stability of the elements of the 

 shell in this gen us, we have observed a very remarkable and 

 probably unique fact in palaeontology. It is that on Plate 1, 

 of Dr. Y. Mojsisovics' u Das (icbirgo um llallstatt," is shown 

 seven species of Triassic Orthoceras, which could be inter- 

 calated among the plates representing the Silurian species of 

 our fauna No. 3, without any man of science supposing that 

 he saw the last remains of that type. In fact, they are hardly 

 distinct from the species which characterized the epoch of the 

 most luxuriant vitality of the genus. 



2. Nautilus has enjoyed the privilege of an incomparable dura- 

 tion, from the first appearance of cephalopods, at the origin of 

 the second Silurian fauna to the present time. The succession 

 of its species has traversed every geological age, and if it had 

 boon subjected to the supposed influences of evolution, it 

 would show us a series of transformations or of progressions 

 which, accumulating, would far remove the present from the 

 primitive forms. A\ r e do not see why these extreme forms 

 arising during the existence of Nautilus should not contrast 



