STRUCTURE OF THE CYSTIDE^. 13 



Scandinavia and Russia* 20 



Great Britainf , 13 



Bohemia, about 8 



Canada 21 



New York 1 



63 



In consequence of the imperfection of the specimens and some 

 confusion in the descriptions of different authors, the above numbers 

 may not be exactly correct; but from what I have seen it appears 

 to me that there are more than sixty species, described and unde- 

 scribed, belonging to this period. 



In the Upper Silurian there are in Great Britain nine species, 

 and in Canada and New York about the same number, but none in 

 either Bohemia or Scandinavia have yet been made public. 



According to the present state of our knowledge, then, in the 

 lower half of the Lower Silurian there are four species, in the 

 upper half sixty-three, and in the Upper Silurian eighteen. 



Very little dependance however can be placed upon numerical 

 comparisons, such as the above, in dealing with questions relating 

 to the Cystidege or Crinoideae, for the reason that new discoveries 

 are every year being made which very materially change the aspect 

 ^of these computations. For instance, six years ago only eleven 

 Crinoids, one Cystidean, and one Star-fish, were known in the 

 Lower Silurian of New York and Canada, but in the collection of 

 the Geological Survey of Canada there are now twenty-one species 

 of Cystideans, about fifty Crinoids, and ten Star-fishes, or in all 

 eighty-one species of Echinodermata from this formation instead of 

 thirteen. 



In the Devonian formation several forms resembling Cystideae 

 have been referred to that group of organisms ; but it remains still 

 to be shewn that they are true Cystideans. The weight of the 

 evidence tends to shew that the race was ushered in with the first 

 living inhabitants of the deep — attained its greatest development 

 in the latter portion of the Lower Silurian era, and died out about 

 the time of the commencernent of the Devonian. Of its associates 

 in the Primordial Zone, the Brachiopoda, Pteropoda and Bryozoa 

 remain to the present day. The trilobites held their possession of 



* Bronn's Index Palaeontologicus. Zweite Abtheilung, p. 181. 



t On the British Cystideae, by Prof. B. Forbes. Memoirs of the Geological Survey 

 of Great Britain, vol. ii. part 2, p. 483 et seq. 



