12 STRUCTURE OF THE CYSTIDE^. 



represented in England by the Llandeilo and Bala or Caradoc 

 groups of Murchison ; in Bohemia by the stage D, containing the 

 "second fauna" of Barrande ; in Scandinavia and Russia by the 

 Regions BC, C and D of Angelin, and the "Pleta" or Orthoceratite 

 limestone ; and in Canada by all the groups from the base of the 

 Calciferous Sandrock up to the top of the Hudson River group. 



While these rocks were slowly being deposited, the Cystideae 

 literally covered the bottom of the ocean in dense swarms in certain 

 localities which were favorable to their existence, one generation 

 growing upon the remains of another, until thick beds were formed. 

 In Russia, Norway and Sweden, Sir Roderick Murchison* discovered 

 them in the Pleta limestone, which appears to be of the age of the 

 Chazy, Birdseye, Black River and Trenton limestones, packed together 

 like "bunches of enormous grapes;" and in Bohemia M. Barrande 

 has found them equally abundant. He says that the Crinoids and 

 Star-fishes have left only insignificant traces, but the Cystideae form 

 entire beds of from one to two yards in thickness.t 



In Canada they make their appearance rarely in the Calciferous 

 Sandrock, but in the Chazy and Trenton their remains are more 

 common, consisting however mostly of the detached plates packed 

 together in thick strata. They are not very generally distributed, 

 but confined to certain localities. Throughout extensive regions 

 occupied by these formations scarcely a vestige of a Cystidean is to 

 be found ; but in other places, such as the neighbourhoods of the 

 cities of Montreal and Ottawa, they are exceedingly plentiful. 

 Everywhere however good specimens are rare. 



M. Barrande, in comparing the European rocks of this age, 

 observes, that in Bohemia the Cystidean zone occurs about the 

 centre of his stage of Quartzites D, which would be also the 

 equivalent of Angelin's group C. In England the corresponding 

 level would be about the Bala limestone, where the principal masses 

 of Cystideae are found. The abundance of their remains in the 

 Chazy and Trenton of Canada confirms the views of M. Barrande, 

 and at the same time tends to shew that these two American 

 formations should be paralleled with the Bala rather than with the 

 Llandeilo. This question however cannot be decided without 

 more perfect lists of fossils than can be at present procured. 



The number of species of Cystideae that occur in this zone are as 

 follows, so far as I can ascertain, in these countries respectively : — 



• Gaology of Russia and the Ural Mouataias ; by Sir R. I. Murchison, page 38. 

 t Systeme Silurien du centre de la Boheme ; par M. Joachim Barrande. page 66. 



