THE SILURIAN PERIOD 



447 



the Ordovician in species, the importance of the race did not 

 diminish. Some improvements in structure were accomplished, and 

 new genera which later 

 became important 

 were evolved. The 

 evolutional changes 

 were doubtless directly 

 or indirectly the result 

 of the changes in en- 

 vironment, which con- 

 sisted in shiftings of 

 the epicontinental seas 

 and the consequent 

 frequent migrations of 

 faunas and struggles 

 between them. Fig. 417. — Silurian bryozoans (B-E), and pteropod 



BrVOZOa. — - The ^) • -^> Tentaculites gyr acanthus; B, Lichenalia concen- 



1 i«i 1 trie a; C, a portion of B enlarged; D, Callopora elegan- 



coral-hke bryozoans mla . E> a portion o{ D enlarged 



(Fig. 417) were less 



important reef builders in the Silurian than in the Ordovician. 



Mollusca 



Gastropods. — Aside from an increase in the number and variety 

 of species with elevated spines and in a somewhat greater abundance, 



Fig. 418. — Silurian pelecypod : A, Pterinea emacerata. Silurian gastropods 

 B and C, two views of Trematonotus alpheus; D, Strophostylus cyclostomus ; E, Platy 

 stoma (Diaphorostoma) niagarense. 

 CLELAND GEOL. — 29 



