﻿<7. D, Walcott — Cambrian System of North America. 149 



Zoologic Resume,. 



Genera. Species. 



Algse --- 3 9 



Spcmgue — - - 6 13 



Hydrozoa 4 5 



Crinoidea - 1 3 



Annelida 2 5 



Brachiopoda 15 6*7 



Lamellibranchiata 1 1 



Gasteropoda -•- — 14 29 



Pteropoda 5 20 



Crustacea 10 15 



Pcecilopoda 31 226 



92 393 



There are 14 genera common to the Lower and Middle Cam- 

 brian ; 15 common to the Middle and Upper Cambrian ; 11 

 common to the Lower, Middle and Upper Cambrian, and 12 

 common to the Lower and Upper Cambrian. 



Of the 52 genera in the Upper Cambrian, 17 are much more 

 strongly represented in the second fauna, viz : Lingula, Orthis, 

 Leptasna, Triplesia, Bellerophon, Euomphalus, Holopea, Ma- 

 clurea, Metoptoma, Ophileta, Pleurotomaria, Hyolithes, Serpu- 

 lites, Amphion, Bathyurus, and Ogygia. 



Of the above genera, Discina, Pleurotomaria, Amphion, 

 Bathyurus and Ogygia are doubtfully referred to the Cambrian. 

 Several other genera pass up into the base of the Lower Silu- 

 rian (Ordovician), but are not considered as at all characteristic 

 of its fauna. 



When an accurate stratigraphic and paleontologic study is 

 made of the passage beds between the Cambrian and Lower 

 Silurian (Ordovician) systems, or the Potsdam and Upper Cal- 

 ciferous formations of the New York and Canadian sections, 

 we shall possess the data upon which to compare the faunas of 

 the two systems. At present it is to a large extent wanting. 



Stratigraphic position of the Upper, Middle and Lower Cam- 

 brian Faunas. — That the stratigraphic position of the Middle 

 Cambrian fauna on the North American continent is below that 

 of the Potsdam fauna is shown by the Eureka and Highland 

 Eange sections, in Nevada, and the Georgia section, in Ver- 

 mont. In Nevada, in two sections unbroken by faulting or 

 folding of the strata and separated by a geographic distance of 

 one hundred and twenty -five miles, the fauna ranges from 

 2,000 to 4,000 feet below strata carrying a typical Upper Cam- 

 brian, or Potsdam fauna. But three species, Protospongia fen- 

 estrata, Acrotreta gemma and Sienotheca elongata are known to 

 pass up to the Upper Cambrian or Potsdam horizon. In 

 the Georgia, Vermont, section, one of the species, Ptychoparia 



