SILURIAN ANIMALS. 



323 



The great number of genera into which this large order is divided 

 is founded principally on the form and sculpturing of the Buckler, the 

 size and form of the Pygidium, the number of the movable segments, 

 etc. The figures below and on the next page will give an idea of some 

 of these forms. 



It is very interesting to observe that a complex mechanism, the 

 compound eye like that of crustaceans and insects of the present day, 

 was already developed even in the early Primordial times. 



Trilobites commenced, as already stated, in the earliest Primordial, 

 continued through the whole Palaeozoic, and then became extinct for- 

 ever. They are therefore entirely characteristic of the Palaeozoic. They 



Fig. 377. 



Fig. 





Fig. 380 « 



Fig. 380. 



Figs. 377-380 a.— Silurian Trilobites : 377. Trinncleus Pons:erardi. 378. Lichas Boltoni (afte; 

 Hall). 379. Acidaspis crosotus (after Meek). 380. Isotelus gigas, reduced (after Hall). 380 a 

 Same, side-view. 



