348 



CAMBRIAN PERIOD 



named Olenellus (Fig. 311, a). Along with representatives of this 

 genus, many other species of various types are found. To the 

 aggregate, the name Olenellus fauna is given, and Olenellus Cambrian 

 is synonymous with Lower Cambrian and with Georgian. It is 

 not to be understood that representatives of the genus Olenellus 



Fig. 311. CHARACTERISTIC CAMBRIAN TRILOBITES: a, Olenellus gilberti Meek; 

 b, Paradoxides bohemicus Boeck; c, Dikettocephalus pepinensis Owen. These three 

 genera are characteristic of the Lower, Middle and Upper Cambrian, respectively. 



are found in the Lower Cambrian everywhere, or that other genera 

 of trilobites are absent. 



Where formations representing the whole of the period are 

 present, the fossils in the middle beds are not the same as those 

 in the lower. At no single plane is there, as a rule, a striking change 

 in species, but in successively higher beds some of the species found 

 below disappear, and new species come in. These changes show that 

 the inhabitants of the sea changed as time went on. At about that 

 stage in the Cambrian system where the genus Olenellus drops out. 

 the genus Paradoxides (Fig. 311, b) appears in some places. The 

 species associated with Paradoxides are somewhat different from 

 those associated with Olenellus. The Paradoxides and their asso- 

 ciates constitute the Paradoxides fauna, which includes many 

 species of other genera of trilobites, and many species not related 

 to trilobites. By general agreement, the Middle Cambrian, on 

 both sides of the North Atlantic, is defined by the Paradoxides 

 fauna, so that Paradoxides Cambrian is synonymous with Middle 



