THE CAMBRIAN PERIOD 483 



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

 the aggregate, the name Olenellus fauna has been given, and Ole- 

 nellus Cambrian is synonymous with Lower Cambrian and with 

 Georgian. It is not to be understood that representatives of the 

 genus Olenellus are found in the Lower Cambrian everywhere, 1 or 

 that other genera of trilobites are absent. 



Where formations representing the whole of the period are 

 present, the fossils in the higher beds are not the same as those below. 

 At no single plane is there, as a rule, a very striking change in 

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

 below disappear, and new species come in, as if to take their places. 

 These variations 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. 359, b) 

 appears on both sides of the North Atlantic basin. The other 

 species associated with Paradoxides are somewhat different from 

 those associated with the genus Olenellus. The Paradoxides and 

 their associates constitute the Paradoxides fauna, a fauna which 

 includes also many species of other genera of trilobites, and many 

 species of other classes of animals 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 Cambrian and with Acadian 

 (p. 476). In the western part of North America, and on the 

 opposite side of the North Pacific as well, the Middle Cambrian 

 does not contain Paradoxides, but Olenoides, and its fauna is 

 known as the Olenoides fauna, but it is none the less distinct from 

 fauna of the Lower Cambrian. 



In like manner the Paradoxides and Olenoides faunas are suc- 

 ceeded by another, known as the Dikellocephalus fauna (359, c), 

 and this fauna characterizes the Cambrian strata above the Middle 

 Cambrian. Geologists have agreed to define the Upper Cambrian 

 as the series of strata carrying the Dikellocephalus fauna. 



It is not to be understood that every species of the Paradoxides 

 fauna is unlike every species of the Olenellus fauna below and the 

 Dikellocephalus fauna above. This is not the case; but so many 



1 Walcott, 'Jour, of Geol., Vol. XVII, 1909. 



