128 



THE LIVING WORLD. 



Silurian (2), however, they began to diminish in 

 number, and have been growing of less importance 

 ever since. To-day, though not so much diminished 

 in numbers as the crinoids, they form a comparatively 

 unimportant group with few genera. The orders 

 of Phyllocarida form the only other important illus- 

 tration. This is a small group of Crustacea, quite 

 abundant in the Silurian, which, with the exception 

 of a single genus (Nebalia), is to-day extinct. 



Q 



Te 

 Cr 



FIG. 18. 



Fig. 1 8. Some groups of animals appeared well de- 

 veloped with the Silurian (2) age, and have continued 

 to exist in undiminished numbers ever since, or 

 have even increased in number and diversity (see 

 Fig. 1 8). This is true of a majority of the orders 

 found in the Silurian. Many of them seem to have 



