192 OUR COMMON BRiTtStI FOSSILS. 



The Cambrian Trilobites, as a rule, differ from 

 their Silurian descendants and representatives in 

 having a large number of rings or segments to the 

 thoracic (or middle) part of the body: The tail part 

 (caudal shield) is, however, less developed than in the 

 Silurian species. The side-lobes of some genera, 

 Paradoxides and Acidaspis, are fringed, and, in the 

 case of the latter, further adorned with spines. Some 

 of these may have been merely sexual distinctions, 

 although we are now forced to regard them as specific. 

 Dean Buckland and many other naturalists regarded 

 an isopod crustacean abundant in the seas around 

 Tierra del Fuego and the Straits of Magellan, as 

 nearly allied to this group of Trilobites. This 

 crustacean is called Serolis. Its cephalic shield has 

 compound sessile eyes, arranged in half-moon-shaped 

 lobes exactly like those of some Trilobites. The 

 segments or joints of the thoracic portion of the body 

 are fringed, as in Paradoxides^ and there is a movable 

 caudal or tail shield, as in Phacops caudattis (Fig. 155), 

 an abundant Silurian Trilobite. Only the antennae and 

 mouth-organs differentiate them. But these are very 

 thin and weak, and after death may soon be detached, 

 as various geologists believe was the case with some 

 Trilobites. The legs are fitted for crawling about, 

 but, as is frequent in animals living in sea-Water, they 

 arc also weak and thin. The Serolis is a slow 

 crawler and swimmer, and is usually found on sea- 

 weed. Some geologists have imagined that a feW 



