GENERAL REMARKS. 



In studying the fossils placed in our hands, from the several Palaeozoic 

 formations occurring in the regions examined by the "Geological Exploration 

 of the Fortieth Parallel", there are several groups of special interest noticed; 

 but perhaps none more deserving of close attention than those from the lime- 

 stones and silico-calcareous shales representing the Potsdam Period, from the 

 White Pine and Eureka mining-districts and from Schell Creek. The peculiar 

 interest attached to these specimens arises chiefly from the great number 

 and variety of the Trilobitic remains, represented in the few blocks present 

 in the collection, and their great resemblance to forms previously described 

 from the soft, friable sandstones of the same period in the Mississippi Valley, 

 within the States of Wisconsin, Iowa, and Minnesota. Although there is 

 not a single species common to the two regions, yet there is such a close 

 generic resemblance as to leave no doubt whatever of the positive identity 

 of the formations. As yet, there have been fewer genera recognized from 

 these far western localities than from the more eastern ones; but if the 

 smaller number of distinct locations and the restricted space or territory 

 over which collections have been made, as well as the small amount of 

 material examined, be taken into consideration, it will at once be seen that 

 the probability of the existence of as varied and equally abundant fauna is 

 very great, especially as there are at present several undescribed, but quite 

 distinct, forms recognized among the material in hand, but in fragments too 

 poor for description or figures. 



The species here described also present a striking resemblance to forms 

 recognized in beds of corresponding age in the San Saba Valley, Texas, by 

 Dr. F. Roemer, in his work on the Cretaceous formations of Texas (Kreid. von 



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