SILURIAN SYSTEM. 95 



mats and sea-pens (graptolites, &c.) ; shell-fish of every order, 

 bivalve and univalve, deep-sea and shore dwellers ; radiate 

 animals, like the encrinites and the star-fishes ; sea-worms in 

 their tracks and burrows ; and crustaceans, chiefly trilobites 

 and eurypterites, having some resemblance and affinity to 

 the existing limulus or king-crab. These organisms are not 

 found indiscriminately throughout the system, but vary in 

 number and distribution according to the kind of stratum 

 in which they are imbedded every zoophyte and shell-fish 

 preferring a certain kind of sea-bottom ; and according as 

 they occur in the lower, middle, or upper portion of the 

 system the upper being the more prolific and characterised 

 by the higher species. Numerous and varied as they are, 

 they are (with the exception of the obscure land -twigs) 

 exclusively marine ; and if we regard the uppermost beds, 

 with their fish-remains, as the base of the Old Eed Sand- 

 stone, they are entirely invertebrate,* and mark, so far as our 

 present evidence goes, the close of a long primary cycle 

 during which vitality was gradually evolving, in a fixed and 

 definite order, from lower to higher .manifestations. 



As economic repositories these primary systems are, in 

 some regions, of considerable importance; less, however, 

 for their rock-products than for the metalliferous veins by 



* The reader must guard against the idea that there are any sharp lines 

 of demarcation between the so-called Systems of geologists. The life of 

 certain estuaries and seas may no doubt be brought to a close by some 

 sudden catastrophe, but such breaks are merely local, and do not affect 

 the general life-arrangements of the globe. When we speak, therefore, of 

 the Silurian as " marking the close of a long invertebrate period," it is not 

 meant to be asserted that there were absolutely no fishes during the de- 

 position of the uppermost Silurians, but simply that the Primary Periods 

 as a -whole were characterised by their want of vertebrate remains, and that 

 the strata in which fish-remains do occur may be regarded, without detri- 

 ment to the science, either as uppermost Silurian or as lowermost Old 

 Red Sandstone. 



