xl 



PROCEEDINGS OP THE GEOLOGICAL SOCIETY. 



there is proof at Noeth-grug of a slight unconformity between them 

 and the black slates on which they lie. 



The Upper Llandovery rocks, however, behave in a very different 

 manner. These range interruptedly from Harloes Bay in Pembroke- 

 shire through Caermarthenshire to Builth, the Longmynd, and the 

 typical Silurian country of Shropshire ; and everywhere they rest 

 quite unconformably on older rocks, lying sometimes on the denuded 

 edges of the Lower Llandovery beds, sometimes on Caradoc Sand- 

 stone, and at Builth and the Longmynd on the highly contorted and 

 denuded Llandeilo and Cambrian strata. 



The lists of the Geological Survey show that about seventy species 

 of fossils are known in the Lower and Upper Llandovery strata. 

 These, taken together, are so distinctive that it has been proposed 

 to elevate the strata in which they occur into a Middle Silurian 

 series ; and, at all events, their stratigraphical and palseontological 

 relations are so important that I give the following analysis of 

 the fossils of the Lower Llandovery beds, followed by more con- 

 densed statements of the palaeontological relations of the Upper 

 Llandovery beds and Wenlock shale. 





Wenlock, 

 &c. 



Upper 

 Llandovery. 



Lower 

 Llandovery. 



Caradoc. 





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Thompsoni 



Calymene Blumenbachii ... 





calligramma 





elegantula 



Atrypa crassa 



reticularis 



marginalia 



Leptasna sericea 



scissa 



transversalis 



Pentamerus oblongus 



laevis 



undatus 



depressa 



Pecten 



Cyclonema crebristria 



Eunema 



Turbo 



Murchisonia Pryceas 



Bellerophon acutus 



bilobatus 



dilatatus 



Conularia 







13 



20 



28 



16 



The foregoing Table shows that, according to existing British lists, 

 there are twenty-eight Lower Llandovery species. Of these, sixteen 



