526 HISTORICAL GEOLOGY. 



Barrande, in his review of the Trilobite Fauna of the Paleozoic, which 

 he published in 1871, made the total number of Cambrian and Silurian 

 species then known 1500 ; and those subsequently introduced, in the Devo- 

 nian and Carboniferous eras, about 200. He states that in the Cambrian 

 period the number of species known was 252 in 28 genera ; in the Lower 

 Silurian, 886 species in 52 genera, eight of these genera being of Cam- 

 brian origin; then in the Upper Silurian — his third Fauna — there were 

 482 species in 20 genera, but only three of these 20 genera were of Upper 

 Silurian origin, the rest already existing in the Lower Silurian. 



The number of known Cambrian species of Trilobites has been increased 

 since 1871 by more than 200 ; and besides, a larger number of the genera are 

 now known to date from the Cambrian. But still Barrande's conclusion 

 remains right — that the Lower Silurian was the era of maximum develop- 

 ment of Trilobites. In North America, the Lower Silurian beds add 215 

 species and 30 genera of Trilobites ; the Upper Silurian only 81 species and 

 three genera ; and of these three, two occur in Europe. The type for awhile 

 was the highest of the seas ; but that of Cephalopods, of later introduction, 

 had passed it in size, grade, and power before the Lower Silurian era closed. 

 Such facts give strong characteristics to the Lower Silurian, and exhibit its 

 contrast to the Upper. 



The Hydrozoans, Actinozoans, and Bryozoans, which usually produce, by 

 multiplication, compound groups of branching and other forms, and show 

 thereby their low grade among species, are rare fossils in the Cambrian as 

 simple individuals, and are wholly unknown in compound groups, although 

 such groups are indicative of low grade, and the Bryozoans are the lowest 

 of the Molluscoids. But in the Lower Silurian era the compound forms 

 after the commencement of the Chazy period were common, and were emi- 

 nently so during the Trenton period. Ulrich states, after an investigation 

 of the Bryozoans of Minnesota (a few of his figures are reproduced on 

 page 506), that the contributions from them of calcareous material for the 

 Lower Silurian limestones of that state were twice as great as those from 

 the Brachiopods {Rep. L. Sil. Bry. Minnesota, 1893). 



UPTURNINGS AT THE CLOSE OF THE LOWER SILURIAN. 



AMERICAN. 



General quiet of the Lower Silurian era. — The strata of the Lower Silu- 

 rian in eastern North America appear to have been laid down, one over 

 the other, without intervening dislocations. Through the era there were 

 extensive oscillations in the Avater level, for this is indicated by the varying 

 limits of the formations, as well as by changes in the kinds of rocks ; and 

 the exposed beds of one period probably suffered much by denudation before 

 the next were deposited. But these oscillations resulted in no great upturn- 

 ings of the rocks. The era was one of quiet progress in sedimentary 



