1S72.] NICHOLSON — migrations of the griptoliies. 229 



be found, however, in the Caradoc strata of Ireland, and in the 

 Hudson-Hiver group of North America. 



F. Saxon and Eohejiian Areas. — I have now endeavoured to show 

 that two great migrations of GraiDtolites took place from the area of 

 the south of Scotland at the close of the Upper Llandeilo period — one 

 -westward through Ireland to America, the other southward into 

 the north of England. I shaU now try to show that a third great 

 migration took place at the same time from the same area, in a south- 

 east direction, into the Silurian seas of Saxony and Bohemia. 



The Graptolitiferous rocks of the " Grauwackenformation " of 

 Saxony are believed by Geinitz to form the summit of the Lower 

 Silurian series ; so that they would correspond pretty closely with 

 the upper portion of the Caradoc series. That the Graptolites of 

 this series were derived from the British area is shown almost con- 

 clusively by the subjoined list of Saxon species identical with British 

 forms : — 



Saxon Graptolites derived from Britain, 



Diplograpsus pristis, His. '' 



palmeus, Barr. 



folium, His. 



Climacograpsus tei'etiusculus, His. 



Graptolites Sagittarius, His. I Speciesderived from the Upper Llan- 



Nilssoni, Barr. \ deilo of the south of Scotland, 



tenuis, Portl. 



lobiferus, McCoy. 



Sedgwickii, Portl. 



Eastrites peregrinus, Barr. 



Graptolites priodon, Bronn. '] „ ■ i- t p .i/-< •, -.i-i, 



coiQnus Barr \ '^P^'-'i^^ derived from the Coniston Miidstones. 



All these species occur in the mudstones of the Coniston series ; 

 but it is tolerably clear that they were not derived from this source, 

 but from an area like the Upper Llandeilo of the south of Scotland, in 

 the seas of which the genus Didymograpsus was represented. Two 

 Didymograpsi, namely, occur in the Silurians of Saxony ; and one of 

 them, at any rate, seems to be identical with a Scotch species. It 

 is also clear that the channel of communication between the British 

 and Saxon areas remained open for a long period, since the Silurian 

 rocks of Saxony contain Graptolites priodon and G. colonus, both of 

 Avhich are wanting in the Upper Llandeilos of Scotland, but appear 

 in the later deposits of the Gala group and in the mudstones of the 

 Coniston series. It is probable, therefore, that more than one migra- 

 tion of Graptolites took place at different periods from the British to 

 the Saxon area. 



Almost all that has just been said about Saxony Avould apply to 

 the case of the Bohemian area ; but I must not omit to notice that 

 M. Barrande has already recognized the British derivation of the 

 Bohemian Graptolites (Defense des Colonies, 1870). We may there- 

 fore regard the stocking of the Saxon area with Graptolites as being 

 merely an early stage in a steady easterly migration of these organisms 

 from British seas towards the close of the Lower Silurian period. 



M. Barrande recognizes two chief Graptolitic zones in the Silurian 



