400 



SILURIAN PERIOD 



Knowledge of Silurian marine plant life is meager. While it 

 must have been abundant theoretically, only obscure markings have 

 been found, and their interpretation is more or less doubtful. 



Late Silurian fauna. Following the luxuriant life of the mid- 

 Silurian epoch, there came, in North America at least, a notable 

 decline, due to the withdrawal of the epicontinental waters from the 



Fig. 354. ReccptaculilesoweniHall; "lead coral" or "sunflower"; original 9^ 

 inches in diameter. 



larger part of the interior, and to the conversion of the remainder into 

 an excessively salt sea, in the deposits of which few fossils are found. 

 At the very close of the period there was in the Salina basin a gradual 

 return of conditions hospitable to life. The fauna of these late 

 Silurian beds is limited, and radically unlike that which preceded. 

 Most of the familiar marine types are absent from the later fauna, 

 and its signal feature is an abundance of arthropods (p. 686) of types 

 barely represented before. The most characteristic of these were 

 the great Eurypterus and the still more gigantic Pterygotus (Fig. 

 355, a and b). The former reached a length of a foot and a half or 

 more, and in the next period the latter attained a length of over six 



