C. D. Waleott — Position of the Olenellus Fauna. 41 



Among the Brachiopoda the genus Obolella has a large 

 development in the Lower Cambrian and is present in the 

 Upper Cambrian, but is scarcely represented in the Middle 

 Cambrian ; and the Brachiopoda, as a whole, are more largely 

 developed in species and number of individuals than in the 

 Middle Cambrian. Lamellibranchs are represented in the 

 Lower Cambrian, but not in the Middle Cambrian. 



Among the Gasteropods, Pleurotomaria and Straparollina are 

 yet to be discovered in the Middle Cambrian. The same is 

 true of the Phyllopod crustacean Protoearis Marshi. 



Viewing the Olenellus fauna as a whole and comparing 

 it with the known Middle Cambrian fauna of the Atlantic 

 Basin, or the Paradoxides fauna, the impression made is that 

 the former is more highly differentiated, and, zoologically con- 

 sidered, the successor of the Paradoxides fauna. If, in our 

 comparison, we include the Middle Cambrian fauna of the inte- 

 rior of the continent, this conclusion will be changed, owing 

 to the presence of a group of trilobites, from the Middle Cam- 

 brian of Nevada, Utah and British Columbia, that includes 

 the genera Olenoides, Asaphiscus, Bathyuriscus, Karlia and 

 Ogygopsis. These genera suggest the trilobites of the second 

 or Ordovician fauna, and serve to connect the Middle Cambrian 

 fauna so closely with the second fauna that the idea of its 

 preceding the Olenellus fauna cannot be entertained. 



It was owing to the comparison made between the two 

 faunas in the Atlantic Basin that led me to so long retain the 

 view that the Olenellus fauna succeeded the Paradoxides fauna 

 in time, and to think that the Paradoxides fauna would be 

 found, if at all, beneath the Olenellus zone, in the interior of 

 the continent. ISTow that we know that the Olenellus fauna 

 occurs beneath the Paradoxides zone in America, and that 

 there is a rej)resentative Middle Cambrian fauna, in the valley 

 of the Hudson, that serves in a measure to connect the Para- 

 doxides fauna of the Atlantic Basin and the Middle Cambrian 

 fauna of the interior of the continent, there is no hesitation in 

 referring the group of species, forming the fauna between the 

 Olenellus and the Upper Cambrian zone, in the Rocky 

 Mountain province, to the Middle Cambrian, and in correlating 

 its stratigraphic position with that of the Paradoxides fauna of 

 the Atlantic Province. 



Comparison and Correlation. — If a comparison is made be- 

 tween the fauna of the Olenellus Zone and that of the Ordo- 

 vician, the superiority of the latter in number of species, 

 genera and families is at once apparent. If the comparison is 

 extended to class characters the disparity between the two is 

 very much reduced and it is made evident that the evolution of 

 life between the epoch of the Olenellus fauna and the epoch 



