284 



HISTORICAL QEOLOGY. 



nally, the upper surface of the shell was divided by two 

 depressions into three lobes (hence the name). On each 



side of the 

 head-shield, in 

 position exact- 

 ly as in the 

 "king-crab (Li- 

 mulus), were 

 placed the eyes; 

 and, strange to 

 say, we find the 

 eye, even at this 

 early time, al- 

 ready a com- 

 plex structure 

 well adapted to 

 form an image 

 (Fig. 195). 

 Recently have 

 been discovered 

 jointed legs, 

 I, I, each with 

 two branches, 

 one for crawl- 

 ing and one for 

 swimming; and 

 also slender, 

 many-jointed 

 antennae, , a, 



FIG. 196. Restoration of upper side of calymene. 

 Beecher.) 



(After 



manv crus- 

 taceans of the 

 present day (Fig. 196). They had the habit, which 

 many crustaceans now have, of folding themselves so as 

 to bring head and tail together in front, as shown in Fig. 

 199. In the following figures (Figs. 197, 198) we give 

 some examples of Silurian Trilobites. 



