DISTRIBUTION OF THE MOLLUSCA IN TIME. 2-21 



these elements, notwithstanding their specific and temporary 

 A'ariations. These facts are manifested above all in the genera 

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

 Nautilus. 



IV. Stability of the Genera. 



1. Orthoceras takes the second rank, by its longevit}', among 

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

 it furnished alone about half those of the Palaeozoic cepha- 

 lopods. It traverses every age, commencing with the origin 

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

 Its species, very diverse in ever}' horizon, preserve neverthe- 

 less their typical features veiy plainlj^, in about 1146 forms 

 enumerated in our studies. 



Among the proofs of the stability of the elements of the 

 shell in this genus, we have observed a very remarkable and 

 probabl}^ unique fact in pala?ontology. It is that on Plate 

 1, of Dr. y. Mojsisovics' " Das Gebirge um Hallstatt," is 

 shown seven species of Triassic Orthoceras, which could be 

 intercalated 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 charac- 

 terized the epoch of the most luxuriant vitality of the genus. 



2.- Nautilus has enjoyed the privilege of an incomparable duration, 

 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 been 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. We do not see why these extreme forms 

 arising during the existence of Nautilus should not contrast 

 among themselves like those of the Amphioxus and of Homo 

 sapiens, between which imaginary evolution has worked, 

 according to theorj'^, in nearly the same period of time. 



But the material facts discovered by paleeontology and 

 accessible to every one, dispel all illusion. In efiect, the 

 Nautilides, notwithstanding the great number of their specific 

 forms, which must exceed 300, depart so little from their 

 initial type, throughout the geological ages, that the merest 

 novice would not hesitate in any case to recognize their 

 generic nature. The variations or differences among the 

 species, oscillate without any tendency to continue in a single 

 direction so as to found a new type. In the actual fauna, 

 Nautilus does not show, between its forms and the primitive 

 forms, any greater differences than those which all naturalists 



