BARRANDE ON THE TRILOBITE FAUNAS. 



35 



of the stratigraphical division corresponding to the Third Fauna, a 

 very surprising analogy of all the fossils is observable, and there are 

 even a great number of species common to all these districts. But 

 the higher we ascend the series, in each district respectively, the 

 number of these analogous and common species diminishes more and 

 more, as if the connexion between the Silurian seas of this period had 

 gradually been interrupted, and disunited basins only ultimately re- 

 mained. Thus the Homalonoti, which characterize the Ludlow series 

 in England, are unknown in Bohemia ; and, on the contrary, a great 

 part of my Third Fauna is not represented either in England or 

 Sweden. In Bohemia, however, the constant predominance of the 

 Trilobites clearly indicates the continuance of the Silurian period. 

 They are accompanied by Brachiopoda, Gasteropoda, &c., some of 

 which are again found in analogous, if not identical, forms in the, so- 

 called Devonian, deposits of the Sarthe Department, in France, and 

 partly also of the Rhine districts and the Hartz. These facts, resting 

 on recent discoveries, which I have found to be confirmed by the 

 examination of my friend De Verneuil's fine collection, give rise to 

 some uncertainty as to the position of the horizon in Europe dividing 

 the Silurian from the Devonian System. In England only this un- 

 certainty does not exist, where the Tilestone beds have been regarded 

 as the hmit of the Upper Ludlow series. 



For my part, I am inclined to look for the estabhshment of this as 

 yet undefined boundary in the consideration of the vertical distribu- 

 tion of the Trilobites. First, I would remark, that the genus Caly- 

 mene never reaches the true Devonian formation. The Calymene 

 tuberculosa, Salter, is quoted by him from the Upper Ludlow beds, in ' 

 England, but does not occur higher up. In Bohemia I find the Caly- 

 mene interject a, Corda, in my Group G, but no higher. In ilmerica 

 Calymene platys. Green, is quoted from the sandstone of Schoharrie, 

 which has been called Devonian, but De Verne uil refers to this with 

 some doubt, and possibly the geological place given for this fossil 

 may be erroneous, since Green says in his description that it was 

 found in a travelled block. We know that remains of the genus 

 Calymene (as defined by Emmerich in his Dissertation, 1839) have 

 [not] * been found in the Devonian rocks on the Rhine, in the Hartz, 

 &c. To this altogether negative character I will add a positive one, 

 viz. the occurrence in all Devonian districts of the Pleuracanthus, or 

 species of Phacops with the pygidium armed with teeth {Ph. punc- 

 tatus^^Ph. arachnoideSy Ph. stellatus, Ph. laciniatus, Ph. calli- 

 teles, &c.), a group that occurs in no well-recognized Silurian rocks, 

 whilst it is found in all Devonian countries, — in Devonshire, France, 

 Spain, on the Rhine, in the Hartz, &c. The oldest formation in which 

 the Pleuracanthus occurs is the Spirifer-sandstone or Older Rhenish 

 Grauwacke of F. Ad. Roemer. This then, according to my view, is 

 the base of the Devonian System, and should be grouped with it, and 

 not, as some think, with the Silurian System. 



By the consideration of the Trilobites alone I have arrived at this 

 conclusion, to which also F. Ad. Roemer was led by the consideration 

 * Query " wie eine," a misprint for *' keine." 



