STRUCTURE OF THE CYSTIDE^. 11 



their appearance upon the surface of this planet. The Lower 

 Silurian formation, in the several countries where it has been most 

 studied, has at its base a great thickness of stratified rocks which 

 are alto2:ether without fossils — at least none have been discovered 

 in them up to the present time. Then follows in conformable 

 succession a series in which organic remains do occur, but not in 

 any great abundance. This is the lower half of the fossiliferous 

 portion of the Lower Silurian. In Great Britain these strata arc 

 the Lingula Flags of Sir Eoderick Murchison ; in Bohemia the 

 Primordial Zone of Barrande ; and in Norway and Sweden the 

 Alum Slates, or Regions A and B, of M. Angelin, the leading 

 palceontologist of that country. In America they have not been 

 distinctly recognized, although it is doubtfully anticipated that the 

 Potsdam sandstone and the lowest sandstones of the western states 

 may be of the same age. It is more probable that some of the 

 ancient schists in the eastern states, where a large trilobite of the 

 genus Paradoxides has been found, are of the age of this "primordial 

 zone of life." In whatever way this point may be decided 

 hereafter, it is only in Bohemia that Cystideas have been found so 

 low down in the geological series. Four species have there been 

 discovered, together with twenty seven species of Trilobites, one 

 Brachiopod (Orthis Romingeri, Barrande), and one Pteropod (Ptigi- 

 unculus primus, Barrande), but no Crinoids. 



In Scandinavia the Primordial Zone has not yet yielded traces 

 of either Crinoids or Cystideae, but seventy-one species of trilobites. 

 and eight Brachiopods of the genera Lhigula, Orbicula, Orthis and 

 Atrypa, have been discovered, with one or two graptolites and a 

 small orthoceratite, near the top.* 



In England the Lingula Flags, which are regarded as the 

 equivalents of the Bohemian and Scandinavian deposits, have 

 furnished a very similar fauna of trilobites and rare mollusca, with 

 one or two graptolites ; but up to this date only a fragment of a 

 crinoidal column and no Cystideans. It is also to be observed, 

 that in none of these countries have any corals been detected in 

 these lowest fossiliferous strata. 



In the upper half of the Lower Silurian, organic remains become 

 exceedingly abundant, and it is in this part of the geological series 

 that the Cystideae attain their greatest development, both in the 

 numbers of the species and of the individuals. This deposit is 



* Parallele entre les Depots Siluriens de Boheme et de Scandinavie ; par M. Barrande 

 page 39 et seq. 



