On the Geological Distribution of the Rhabdophora. 367 



upwards into the Riccarton Flags, which are the South- Scot- 

 tish equivalents of the Wenlock beds of Siluria. 



This also appears to be the general zoological character of 

 the Tarannon shale of Wales and the north of England, 

 which occupies the systematic place of the Gala group, to 

 which, however, it is vastly inferior in vertical thickness and 

 in the richness and variety of its Graptolite fauna. 



In the Gala beds of the south of Scotland we recognize at 

 present two main divisions. In the Lower Gala alone do we 

 meet with the survivals from the Birkhill fauna ; but these 

 are here associated with the typical Gala species, Mono- 

 graptus exiguus, Nich., M. galaensis, Lapw. — together with 

 M. priodon, Bronn, which ranges upwards from this horizon 

 into the Wenlock shale. In the Upper Gala the fauna gives 

 many indications of the gradual change into that of the Salo- 

 pian type — Retiolites Geinitzianus, Barr., Monograptus ric- 

 cartonensis. and other Wenlock forms being frequently met 

 with. Here we meet for the first time with the genus Cyrto- 

 graptus of Carruthers. The genus Rastrites, however, is 

 already extinct ; and all the Diplograptidas have disappeared, 

 with the exception of an occasional form in the basal beds, 

 minute and hardly capable of specific identification. 



The graptolitiferous Tarannon shales of Conway yield a 

 fauna corresponding to that of the earlier portion of the Gala 

 beds; and the few forms hitherto collected from the Gala 

 shales (Knock beds) of Westmoreland and the Tieveshilly 

 shales of North-eastern Ireland afford unmistakable indications 

 of the presence in these localities of a corresponding assemblage 

 of forms. 



The predominant and characteristic fossil of the Lower 

 Gala subformation is Monograptus exiguus, Nich., which is 

 as yet unknown outside the limits of the Lower Gala- 

 Tarannon series. 



Salopian or WenlocJc-Ludlow Formation. 



In the Valentian or Llandovery-Tarannon formation, as we 

 have seen, the Diplograptidse, so prevalent in its earlier strata, 

 succumb before the swift increase of the Monograptidse, 

 till finally in the highest Tarannon beds they have dwindled 

 away to an occasional diminutive form of Diplograptus only. 

 In the overlying Salopian beds even these degenerate forms 

 appear to be wanting; and in the lower divisions of the Wenlock 

 shales of Britain the numerous Monograptidse are accompanied 

 only by the single diprionidian species Retiolites Geinitzianus } 

 Barr. This, again, is unknown in the highest beds of the 



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