SILURIAN ERA. 



87 



teropods are the predominating forms in existing waters.* 

 In the articulate division we have numerous annelid mark- 

 ings the trails and burrows of sea- worms; the calcareous 

 crusts and shell-like cases of serpulcv and spirorbes; and 

 a vast and characteristic display of tnlobites (three-lobed), 

 a form of crustacean almost restricted to the period ; toge- 

 ther with the larger and higher forms of eurypterites (broad- 

 fins in allusion to their paddle-like swimming limbs). These 



SILURIAN CORALS AND ECHINODERM3. 



1, Heliolites; 2, Catenipora ; 3, CyathophyUum ; 4, Taxocrinus ; 5, Cystidea ; 6, Palaeaster 



trilobites, along with some smaller bivalved forms of crus- 

 tacea, have been long and familiarly known ; but the euryp- 



* We abstain, in this as in other instances of comparison, from nume- 

 rical tabulations, as every year of further discovery and nicer discrimi- 

 nation of species disturbs, if not destroys, the value of such statistics. 

 Not many years ago the Brachiopoda were supposed to be on the very 

 verge of extinction, and yet the application of the dredge to deeper 

 waters has revealed the existence of nearly a dozen genera in modern 

 seas. Every year, too, discovery adds some new form to our lists of 

 fossils, while former lists of so-called species Continental, British, and 

 American are being examined with more rigorous care, and reduced to 

 their proper value. 



