PALEOZOIC ROCKS AND ERA. 



285 



Trilobites are found in great numbers, of almost infinite 

 variety of form and markings, and of size varying from a 

 fraction of an inch to twenty inches in length (Fig. 197). 



FIG. 197. 



FIG. 198. 



FIG. 199. 



FIGS. 197-199. Silurian trilobites: 197. Paradoxides Harlani, x (after Rogers). 

 American. 198. Calymene Blumenbachii. 199. Same in folded condition. 



They come in with the earliest Primordial, reach their 

 maximum in Mid-Silurian, but continue through Palae- 

 ozoic, and pass out forever. They are, therefore, entirely 

 characteristic of the Paleozoic, and especially abundant 

 in Silurian. Although belonging to a distinct order, 

 different from any now living, yet they were more nearly 



