THE CAMBRIAN PERIOD. 241 



in successively higher beds some of the species found below disappear, 

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



Where the full system of the Cambrian is presented, the Olenellus 

 fauna is found to be restricted to its lower portion. So strictly is this 

 true, that (with the possible exception of Newfoundland, see p. 245) 

 the rocks containing the Olenellus fauna are regarded as constituting 

 the Lower Cambrian. Olenellus Cambrian therefore becomes synon- 

 omous with Lower Cambrian and with Georgian. 



At about that stage in the Cambrian system where the genus Olenel- 

 lus drops out, the genus Paradoxides appears; this is at least true for 

 Newfoundland and Europe. The other species of fossils associated 

 with the genus Paradoxides are somewhat different from those asso- 

 ciated with the genus Olenellus, and many of them have a wider range 

 than the Paradoxides. The Paradoxides and its associates constitute 

 the Paradoxides fauna, a fauna which includes 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 of 

 the regions specified is defined by the Paradoxides fauna, so that Para- 

 doxides Cambrian, Middle Cambrian, and Acadian are synonomous 

 terms. The Paradoxides fauna is so unlike the Olenellus fauna that the 

 two may be distinguished by those familiar with fossils. In the western 

 part of North America, the Middle Cambrian series does not contain 

 fossils of the genus Paradoxides, but its fauna is none the less distinct 

 from that of the Lower Cambrian series. 



In like manner the Paradoxides (or other Middle Cambrian fauna) 

 is succeeded by another, known as the Olenus or Dikellocephalus fauna, 

 and this fauna characterizes the series of strata which overlie the Middle 

 Cambrian. Geologists have agreed to define the Upper Cambrian as 

 the series of strata carrying the Olenus or Dikellocephalus fauna. This 

 definition is found to be applicable to Europe as well as to America. 



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

 other Middle Cambrian fauna is unlike every species of the Olenellus 

 fauna below and the Olenus fauna above. This is by no means true; 

 but so many of the forms of the three faunas are different, that with 

 a considerable number of species to judge from, their separation is 

 possible. 



In the discrimination of any of these faunas an analogy with 

 living animals is suggested. The present faunas of North and South 



