M. Barrande on the Silurian Si/ stem of Bohemia. 117 



known region. This colony, with the matter in which it is 

 imbedded, has, he thinks, come from the north-east, whereas 

 the materials and the tribes characteristic of the earlier beds 

 have come from the south-west. But if this view be admit- 

 ted, it follows that, at the time the lower Silurian strata of 

 Bohemia, with their characteristic forms, were deposited in 

 that country, other strata, with an entirely distinct group of 

 life, were forming in other parts of Europe. This result 

 leads to questions of high interest to the geologist, — ques- 

 tions which are thus stated : 1^^, How far can palsBontolo- 

 gical resemblance or identity demonstrate that formations 

 geographically isolated from each other, are contempora- 

 neous ? 2d, How far does the dissimilarity between the 

 faunas of isolated and distant basins correspond to the dif- 

 ference in the epoch of the deposits in which they are en- 

 tombed ? 



The numerous races that peopled the seas of this period 

 have almost entirely disappeared at its close. Several of the 

 genera, indeed, continue, but only 21 of the species pass into 

 the next higher group. Of the 220 Cephalopods, not one re- 

 occurs in the following stage. And yet the two deposits are 

 perfectly conformable, and their limit almost imperceptible, 

 as if no great or sudden revolution had intervened. 



The middle calcareous stage F differs, however, in mineral 

 character, from that below. The fetid limestones have disap- 

 peared, and the colour of the rocks is less dark, frequently 

 approaching to white. The calcareous spheroids have also 

 vanished, though the limestone is often nodular, as shewn 

 where it decomposes. It now likewise contains much silex, 

 either disseminated through the rock or collected in irregular 

 cherty masses. 



Its fauna, too, varies much. The vital power of most 

 classes seems to have decreased. The variety of generic 

 types has notably declined, and still more the number of 

 species. Of the Trilobites, there are ten genera, but with 72 

 species, the most numerous being in the two types Proetus 

 (25 species) and Bronteus (19 species), both eminently cha- 

 racteristic of this stage, and next in Phacops (8 species.) 

 Two species of Bronteus, Br. palifer and Br. campanifer, also 

 equal in size the gigantic Asaphi and Paradoxides of the 



