EVOLUTION OF EARLY PALEOZOIC FAUNAS 35 



tions were favorable for a varied fauna. The arenaceous beds (with 

 ripple-marks and trails) of the western Nevada- California area and 

 the interformational conglomerates of eastern New York prove the 

 presence in both areas of relatively shallow water. 



The Olenellus thompsoni fauna/ of late Lower Cambrian time, is 

 widely distributed about the margins of the continental area. Begin- 

 ning at the Straits of Belle Isle on the northeast, it has been found in 

 eastern Massachusetts, western Vermont, eastern New York, eastern 

 Pennsylvania, and along the Appalachian area as far south as central 

 Alabama. In the Cordilleran area it is known to extend from Inyo 

 County, California, to the Wasatch Mountains of Utah, and north- 

 ward to the line of the Canadian Pacific Railway in British Columbia. 



With the exception of vertebrates, echinoderms, and cephalopods, 

 the class-characters of the early Lower Cambrian fauna of Nevada were 

 well advanced toward the varied and rich fauna of the lower Ordo- 

 vician. 



CONDITIONS DURING MIDDLE AND UPPER CAMBRIAN TIME 



The physical conditions of the late Lower Cambrian time continued 

 into early Middle Cambrian time, followed during Middle Cambrian 

 time by a gradual submergence through erosion and probable warping 

 of the surface of most of the continental area south of the Great Lake 

 region.^ As the marine waters slowly encroached upon this great 

 area and upon the shores adjoining the Appalachian and Cordilleran 

 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. 



1 Ihid., p. 569. 



2 Am. Jour. Set., Vol. XLIV, 1892, pp. 56, 57. 



3 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, Neoleniis Matthew, Dorypygella Walcott, Damesella Walcott, 

 Blachwelderla Walcott, Zacanthoides Walcott, and Kootenia Walcott. 



