554 



GEOLOGY 



Sea-worms are recognized through their jaws, tracks, ;m<! burrows. 

 and by the calcareous tubes which some of them secrete* I. 



The vertebrates. In the earlier and mid-Silurian deposits feu- 

 relics of fishes have been found, and these few are very imperfect; 

 but in the upper part of the system their remains are not rare. 



Marine plants. Knowledge of marine plant life remain 

 before, unsatisfactory. While theoretically it must have been 





Fig. 405. Rcceplaculites oweni Hall ; " lead coral " or " sunflower;" original 

 9J4 inches in diameter. 



abundant, only obscure markings of the fucoidal type and a few 

 evidences of higher forms have been found, and their interpretation 

 is more or less doubtful. 



Foreign Faunas and Migrations 



Deferring the consideration of the peculiar closing fauna for 

 a moment, the relations of this rich and varied mid-Silurian assem- 

 blage may be noted. The general progress of life on other conti- 

 nents, so far as known, was similar to that on the American, but 



