THE SILURIAN PERIOD 



353 



spines, tubercles, and horns which they produced seem to 

 have had little or no real value in their life activities. We 



FIG. 359. An unu- 

 sually spiny trilo- 

 bite (Acidaspis) from 

 the Silurian of Bo- 

 hemia. 



FIG. 360. Atrilobite 

 (Lichas) of the Si- 

 lurian period. Com- 

 pare with Cambrian 

 trilobites. 



FIG. 361. A highly 

 specialized Silurian 

 trilobite of peculiar 

 form (Deiphon). 



shall see in studying the later periods that similar eccentric- 

 ities mark the fall of other groups, such as the ammonites 

 and the reptiles. 



Among the rising groups 

 only a few require special 

 mention. The corals 

 show an increase in the 

 number of composite 

 types, such as the " honey- 

 comb coral" (Fig. 362) 

 and the " chain coral " 

 (Fig. 363), as against the 

 horn corals. Although 

 the echinoderms had been 

 represented even as early 

 as the Cambrian and had attained some importance in the 

 Ordovician, they did not reach commanding prominence until 

 the Silurian. The clear, shallow seas in which the Niagara 

 ooze was produced furnished congenial life conditions not only 

 for corals but for communities of the crinoids (Fig. 364), 



FIG. 362. A piece of honeycomb coral 

 (Favosites). 



