EVOLUTION OF EARLY PALEOZOIC FAUNAS 201 
seas the marine life of the times met with conditions favorable to a 
large development. This is illustrated by the abundant and varied 
Paradoxides fauna on the Atlantic side and the equally varied Pacific 
basin Olenoides' fauna found in nearly all localities where the Middle 
Cambrian sediments were deposited. 
EVOLUTION OF FAUNAS 
That the environment of the faunas of Middle Cambrian time was 
more favorable for their rapid evolution than that of Lower and Upper 
Cambrian time is strikingly shown by the stratigraphic distribution 
of the brachiopods. In the restricted waters of Lower Cambrian time 
the known brachiopods (of the entire world) were represented by 20 
genera and 75 species. In the expanding seas of Middle Cambrian 
time 31 genera and 243 species are known to have existed. With the 
more uniform conditions of Upper Cambrian time, and the dying-out 
of the impulse to variation created by both favorable and unfavorable 
environments in Middle Cambrian time, the brachiopods decreased in 
variety and numbers, and are represented by only 23 genera and 137 
species. 
About the same relative numerical ratios are exhibited by the 
trilobites but the exact statistics are not yet available. The favorable 
environment of the Middle Cambrian fauna is well illustrated by the 
development of Ogygopsis, Asaphiscus, and Bathyuriscus of Cordil- 
leran Middle Cambrian time,? genera which are so far in advance of 
contemporary trilobitic genera that they have sometimes been referred, 
upon biological grounds, to the Upper Cambrian.3 
The closing of Cambrian time was accompanied and followed by 
changes in the relations of the sea and land upon the continental plat- 
form that were favorable, like those of Middle Cambrian time, to the 
t The Olenoides fauna is found on both the eastern and western sides of the 
northern Pacific Ocean, and the Paradoxides fauna on both sides of the northern Atlantic 
Ocean. This fauna includes a group of trilobites that are represented more or less 
fully in the Middle Cambrian rocks of North America, east of the Atlantic basin 
Paradoxides faunas, and in eastern Asia. The fauna includes: Olenoides Meek, 
Dorypyge Dames, Neolenus Matthew, Dorypygella Walcott, Damesella Walcott, 
Blackwelderia Walcott, Zacanthoides Walcott, and Kootenita Walcott. 
2See Bull. U. S. Geol. Survey, No. 30, 1886, Pls. XXX, XXXI, and Canadian 
Alpine Journal, Vol. I, No. 2, 1908, Pl. 3. 
3G. F. Matthew, Trans. Roy. Soc., Canada, 2d ser., Vol. V, 1899, p. 64. 
