166 



been mncli warmer, probably the result of a different 

 distribution of" laud and water, that only assumed its 

 present configuratiou towards the dawn of the modern 

 period. 



Adopting the classification of Sir Roderick J. Murchison 

 and the government geologists, the Cambrian system, 

 represented by an immense thickness of deposits, contains 

 only the remains of Bryozoa and traces of Annelida, the 

 former found in Ireland only. If we adopt the nomencla- 

 ture of Professor Sedgwick, the Cambrian system yields 

 Zoophyta, Annelida, Crustacea, Bryozoa, and Brachiopoda; 

 but, according to the former arrangement, that most 

 generally accepted, these classes appertain to Lower Silu- 

 rian rocks, in which are also found Amoryshozoa, Echino- 

 dermata, Monomyaria, Dimyaria, Pteropoda, Gasteropoda, 

 and Cephalopoda. The Crustacea and Brachiopoda are the 

 most numerous. In the great Silurian system all the marine 

 classes of animals, excepting Cirrhipedia, are undoubtedly 

 present. The Cephalopoda,* the highest class of inver- 

 tebrata, existed in the Lower Silurian, though not in the 

 lowest zone ; and at the latter part of the epoch, jiidging 

 from their great size, they appear to have performed the 

 duties of fishes. The Silurian was a long period abound- 

 ing with invertebrata, when all those classes of life Avere 

 spread over the earth : we find their remains by millions, 

 yet the most careful searches have failed to produce 

 even a trace of a vertebrated animal. 



During the deposition of the Silurian rocks collectively 

 there appears to have been no fishes. The Cephalopoda 

 were then the most highly organized beings that existed, 

 until just at the close of the period, in the upper Ludlow 

 shales, Ave find the first traces of fishes.f They are ex- 

 ceedingly rare, and consist of teeth and fragments of 

 about seA-^en species of Placoids. 



* Lituites and Orthoceras. f Oiichus, Sphagotus, &e. 



