The Era of Invertebrates 



Silurian period, long though it may be in ac- 

 tual years and rich though the rocks are in spe- 

 cies, with the remark that it seems in the main 

 to have been a time of rest, when life developed 

 along lines already laid down without branch- 

 ing out into any new forms. 



With the advent of the Devonian we find 

 the trilobites running riot in curious shapes, be- 

 decked with spines, a decoration perhaps fore- 

 shadowing their coming extinction. For Dr. 

 Beecher considers it a dangerous symptom for 

 animals to suddenly develop spines, and brings 

 forward many instances to show that there is 

 reason to regard this as indicating degeneration, 

 and that it seems to occur among animals when 

 their race is almost run and they are about to 

 disappear from the scene."^ 



Crinoids are still increasing in numbers and 

 beauty, but the nautiloid branch of cephalopods 

 has already begun to decline, although the 

 most beautiful members of the group, the Am- 



* See The Origin and Significance of Spines, by Charles E. 

 Beecher, American Journal of Science for 1898. A series of four 

 papers commencing with July. 



75 



