Remews — Barrande's Cephalopoda, 



3o 



seas was equally independent of the revolutions of the globe and 

 the variation in the nature of the sediments. 



With a view of showing the vertical distribution of the above ten 

 genera of Silurian Cephalopoda of Bohemia, it will be necessary to 

 give the divisions of the strata adopted by M. Barrande : 

 Stages A, B, C, D, Inferior division ) of the 



Stages E, F, G, H, Siiperior division j Silurian System. 

 The stages A and B are considered as the Azoic base of the Silurian 

 ajj^stem, and consists of crystalline, argillaceous and silicious schists, 

 and conglomerates. The stage G, or Primordial zone, marked by 

 peculiar Trilobites, is the equivalent of the Lingula-flags of Eng- 

 land. The stage D, or Quartzites with schists, the equivalent of the 

 Caradoc and Bala, constitutes the second fauna, as nearly all the 

 genera of Trilobites of the first fauna are wanting, and new forms 

 appear both of Trilobites and MoUusca. The stage E, or third 

 fauna, is the most rich of all the divisions, as it contains the maxi- 

 mum of development of genera and species both of Trilobites — of 

 which six new genera appear, with eleven of the previous zone, and 

 between 200 and 300 species of Cephalopods, and nearly 100 Gas- 

 teropoda, besides many Brachiopods, Corals, and Graptolites, the 

 latter disappear in this zone. The stage F, or fourth fauna, shows 

 an evident decrease, the Trilobites being reduced to ten genera, 

 and the Cephalopods, Gasteropods, and Acephala affording but few 

 species ; but the Brachiopods attain their maximum of specific 

 forms. In this stage the Goniatites and Gyroceras first appear. The 

 faunas of G and J3" are considerably reduced, being represented by a 

 very few species, the Trilobites being most abundant, and belonging 

 to the same genera as in the previous period. The last four stages, 

 E — H, are considered to be the representatives of the Upper Silu- 

 rian series. 



Vertical Distribution of the Silurian Cephalopods of the first series, 

 in Bohemia. 



