200 



GEOLOGICAL DISTRIBUTION. 



Silurian period, which many authors are in the habit of insisting 

 upon. Thus, if we take the very elaborate tables of Mr. Etheridge "' 

 as our guide, we find that the rise and fall of the British trilobitic 

 fauna has been, on the whole, gradual, the greatest break occurring 

 between the Llandeilo and Caradoc on the one side, and the Cara- 

 doc and Llandovery on the other, or along the horizon which, by 

 many geologists, is considered to mark a '' Middle " Silurian divi- 

 sion. The following scheme will exhibit the numerical, generic, and 

 specific values presented by the different horizons of the Paleozoic 

 series, from the base of the Siliu-ian to the Carboniferous, inclusive : 



F0KMATI0N8. 



Arenig 



Llandeilo 



Caradoc-Bala 



Lower Llandovery 

 Upper Llandovery. 



Wenlock 



Ludlow 



Devonian 



Carboniferous 



In the Bob.emian basin, where the transition between the Cambrian 

 and Silurian faunas is very abrupt, the decline is, if anything, still 

 more gradual than in Great Britain. Barrande's Etage C (Cam- 

 brian) holds, according to its illustrious monographer, 27 species of 

 trilobites; fitage D (base of Silurian), 118 species; E, 83; F, 88; 

 and G and H together, 66.* 



In the case of the ammonites the disappearance is somewhat 

 more rapid than with the trilobites ; but even here it is not nearly 

 so abrupt as it is very generally conceived to be. Thus, if we take 



* .J. Barrande, "Trilobites," Prajrue, 1871. Of the 66 species contained 

 in faunas G and 11, G alone possesses 64, and H only 2 ; it might, therefore, 

 be assumed tliat the extinction was here very rapid. But, as M. Barrande 

 himself informs us, the deposits of Etage H are of insiirnificant development 

 when compared with either E, F, or G, and have, in fact, practically disap- 

 peared throucrh erosion from the greater portion of the territory which they 

 formerly covered. " Systeme Silurien," i., 1851, p. 81. 



