In Silurian and Devonian Epochs hi 



advanced to ultimate gigantic size and amid freshwater 

 environment, as the true Eurypterida. Another early 

 evolved amid increasingly estuarine and later littoral sur- 

 roundings as the Trilobita, while a third passed on to land 

 and gave origin amongst others to the Scorpionida. Now 

 largely on account of the intermixture or abundance in 

 intercalated strata of the entire or broken shells of de- 

 pauperate examples of Lingula^ more rarely of Chonetes, 

 Clarke and Ruedemann in their elaborate memoir {6^) 

 on the Eurypterida have concluded that these began as 

 marine organisms, and in late Silurian or Devonian time 

 became freshwater. The suggested evidence for a marine 

 life at any period is so scant or wholly wanting, and the 

 many facts in favor of a freshwater origin are so impres- 

 sive, that we accept the latter as true, and trust later to 

 demonstrate this fully. 



As regards the abundance in individuals of the genera 

 of fishes already cited, this must have been prodigious over 

 wide areas. For the widespread continuity and similarity 

 of the Silurian bone-beds of America, of Britain, of Russia, 

 and of Sweden indicate that innumerable myriads must have 

 died within a few hours, and been sealed up entire and 

 preserved from subsequent destructive action; or after a 

 greater or less degree of disintegration, lasting possibly 

 over two or three weeks, the hard calcareous teeth or 

 plates or bones became piled together in layered masses. 



The three groups of animals then, given in the above 

 study, whose remains are specially abundant are the cerati- 

 ocarid crustaceans, the eurypterids, and the fishes. The 

 possible origin of the oil or petroleum yielded by the Silu- 

 rian and later rock-strata has already been discussed. By 

 far the most likely source of such seems to be the animals 

 just named, and especially the fishes. 



Finally as to the affinities of the Silurian fishes, all so 

 far as known belong to very primitive and now wholly ex- 

 tinct groups that are treated of later. The internal tissues 

 were undoubtedly soft and easily perishable; and even if 

 cartilage was present it also must have been of soft con- 

 sistence. So our knowledge of them is wholly confined to 

 the exo-skeleton, which in very primitive types like The- 



