204 PALAEOZOIC TIME — LOWER SILURIAN. 



have only their beginning here; they have a much larger repre- 

 sentation in species during later periods. But the genera of Trilo- 

 bites are in a large degree peculiar to the Primordial. Paradoxides, 

 Peltura, Dicellocephalus, Menocephalus, Arionellus, Conocephalus, have 

 their whole existence in the Potsdam period. The first two are 

 eminently Primordial in character. The others approximate more 

 to genera of the next period. 



The correctness of uniting the Potsdam and Calciferous epochs 

 in one period is apparent in the number of fhese Primordial genera 

 of Trilobites which the two epochs have in common. Paradoxides 

 and Peltura are alone in being confined to the first epoch. The 

 others mentioned have species in both, and there are quite a 

 number of Calciferous species of Dicellocephalus, Bathyurus, and 

 Conocephalus. 



The equivalency or synchronism of the European and American 

 Primordial period is looked for, not in the sameness of species, — for 

 none are known to be common to the two continents, although one 

 or two are so suspected, — but in an identity in a prominent part of 

 the genera. In accordance with this, we find the life of the world 

 on both sides of the Atlantic commencing with species of Lingula, 

 Obolus, Orthis, Theca, Scolithus, and the Trilobites, Paradoxides, 

 Peltura, Conocephalus, Arionellus, Agnostus, and others. 



But, while a general equivalency is apparent, there are marked 

 peculiarities, which are brought out especially in the later part of 

 the American Primordial. There are Gasteropods (Pleurotomarise) 

 and Cephalopods (Orthocerata) even in the earlier epoch of the 

 American Primordial, while no species of either group have been 

 reported from any part of the European ; and in the later Ame- 

 rican epoch, Gasteropods and Cephalopods are represented by many 

 species of several genera. From this great discrepancy it is natural 

 to conclude that the American Primordial period was continued on 

 in time beyond the European. 



The Calciferous formation has an exact representative in Great Britain 

 in the Sutherland limestone of the Northwestern Highlands, among whose 

 fossils are species of Opkileta and others characteristic of the beds. The Calci- 

 ferous beds of America are separated by some geologists abroad from the 

 Primordial or Potsdam period ; but, as appears above, this is not sustained by 

 investigation. 



Should it be proposed to divide the beds differently, and transfer the Dicello- 

 cephalus strata of the Upper Mississippi to the Calciferous, in order to remove 

 ihese Trilobites from the Potsdam, — or to transfer the lower limestone of the 

 Quebec group to the Potsdam, so as to place all the Dicellocephali in the latter 

 (which would thus be done), — objections would be equally encountered. (1.) No 

 break in the strata sustains such a course; (2.) The Calciferous and Potsdam 



