404 CAMBRIAN AND SILURIAN IN NORTH AMERICA. [XV. 



As early as 1842, Professor Hall, in a comparison of the 

 lower palaeozoic rocks of New York with those of Great Britain, 

 declared the Potsdam to be lower than the base of the Upper 

 Cambrian or Bala group of Sedgwick. In 1847, as we have 

 seen, he extended this observation to the Calciferous and 

 Chazy, both of which he placed below this horizon ; which 

 until a year or two previous had been looked upon as the base 

 of the palaeozoic series in Great Britain, and was subsequently 

 made the lower limit of the second fauna of Barrande. Al- 

 though from these facts it was probable that these lower 

 members of the New York system might correspond to the 

 primordial fauna of Barrande, we still remained, in the lan- 

 guage of Professor Hall, without " the means of parallelizing 

 our formations with those of Bohemia, by the fauna there 

 known. The nearest approach to the type of the primordial 

 trilobites was found in the Potsdam of the northwest, de- 

 scribed by Dr. D. D. Owen ; but none of these had been 

 generically identified with Bohemian forms, and the prevailing 

 opinion, sanctioned, as I have understood, by Mr. Barrande, 

 was that the primordial fauna had not been discovered in this 

 country until the rediscovery (in 1856) of Paradoxides Harlani 

 at Braintree, Massachusetts. The fragmentary fossils published 

 in Vol. I. of the Paleontology of New York, and similar forms 

 of the so-called Taconic system, were justly regarded as in- 

 sufficient to warrant any conclusions." (Amer. Jour. Sci. (2), 

 XXXI. 225.) Such, according to Prof. HaU, was the state of 

 the question up to 1860. The Conocephalus, detected by him 

 from the Red sand-rock of Vermont, in 1847, and subse- 

 quently recognized in Europe as an exclusively primordial 

 type, seems to have been forgotten by Hall, and overlooked by 

 others, until it was rediscovered in the sand-rock by Billings 

 in 1861. He had previously, in 1860, detected the same 

 genus at Point Levis, together with Arionellus, and other 

 purely primordial types. Associated with these, and with 

 many other trilobites belonging to the second fauna, were 

 found several species of Dikellocephalus and Menocephalus, 

 genera first made known by Owen from the Potsdam of "Wis- 



