a \. 



(II itf Of //' ,S7/r//. 



The particular study of each of the element- of the -hell of 

 the cephalopod-. of which we have pre-ellted a IV-lline. -I; 



.that none of them ha\e followed a LiT:idn:il variation in any one 

 direction. On the contrary, we have ob-erved the ,-tahility of 

 the-e elements, not withstanding their s|)ecilic and temporary 

 variation-. These facts are manifested above all in t he genera 

 of which the duration has been t he longest . as ( )rt hoceras and 

 Naut ibis. 



IV. stuhilihj of tin' Genera. 



1. Orthoceras take> the second rank, by its longevity, amon- 

 the generic types, but tin- lirst by its richness in species, since 

 it furnished alone about half those of the I'ala-o/oic 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 nevert hele-- 

 their typical features very plainly, in about 1 1 4C> forms 

 enumerated in our studies. 



Amoiiir the proofs of the stability of the elements of the 

 shell in this MVUIIS. we have observed a very remarkable and 

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

 of \h-.Y. Mojsisovic-' Das (Jebiro-e um Hallstalt." is shown 

 seven -pecio of Triassic Orthoceras. which could be inter- 

 calated amonir the plates representing the Silurian species of 

 our fauna No. ?>. without any m:in of science supposing that 

 he -aw the hist remains of t hat type. In fact . t hey are hurdlv 

 distinct from the species which characterized the epoch of 1 he 

 most luxuriant vitality of the irenus. 



'2. Nautilu> has enjoyed the privilege of an incomparable dura- 

 tion, from the fust appearance of cephalopods. at the origin of 

 the second Silurian fauna t o t he present time. The sncc. 

 of its -pecies has tr:i versed every ^eolo^ical a^e. and if it had 

 been subjected to the supposed intlneiices of evolution, it 

 would show us a -die- of transformation- or of pro^ ;iv--ion- 

 Which, accumulating, would far remove the pre.-ent from the 

 primitive form-. \Ve do not -<>ewhythe-e extreme forms 

 duriiiii' the existence of Nautilus should not contrast 



