26 PICTURESQUE SKETCHES 



the great series of the lower silurian rocks, is the ap- 

 parent want of fishes, and, indeed, of all vertebrated 

 animals. We find everywhere abundant proof that 

 these great thicknesses of mud and sand, with occa- 

 sional bands of imperfect limestone, were formed at 

 the bottom of water, and at various depths ; some, 

 in all probability, in shallow water near land, and 

 others in the deepest recesses of the ocean ; but no- 

 where throughout their wide spread in all parts 

 of the world have they yet yielded the smallest 

 fragment that could be referred to a fish. It is, 

 therefore, pretty clear, either that fishes had not then 

 been created, or that the conditions for their develop- 

 ment were so unfavourable that they were extreme- 

 ly rare, and formed no important group among the 

 inhabitants of the sea, in places where other organic 

 remains, often found in newer formations accompany- 

 ing fishes, are very abundant. The animals we do find 

 consist of certain sea- weeds, called Graptolites* the 

 habitation, probably, of compound creatures, )* which 

 seem scarcely to deserve the name of animals ; of other 

 polyps of somewhat higher organization, building those 

 singular and lasting monuments, the coral-reefs ; of 

 animals removed yet another step in advance, and 

 called Crinoids J , and of a singular and extensive 

 group of crustacean animals, known by the name of 

 Trilobites. They also include a considerable group 



* Graptolites, ypairroQ (graptos), written upon ; \iQoQ (lithos), a 

 stone from their appearance. 



-J- Allied to the recent family of Sertularida. 



$ Crinoids, tcpivog (crinos), a lily ; eidog (eidos), resembling. Lily- 

 shaped animals. 



Trilobites, three-lobed, so called from their shape. 



