410 Geological Society : — 



tricts, and containing species of JEglina, Ogygia, Trinucleus, 

 and the well-known graptolites Didymograptus Murchisoni and 

 Diplograptus foliaceus, &c. 



5. Middle Llandeilo. Calcareous slates and flags with the fossils 

 Asaphus tyr annus, Trinucleus Lloydii, Calymene cambrensis, &c. 



6. Upper Llandeilo. Black slates and flags, with the fossils 

 Ogygia Buchii, Trinucleus Jlmbriatus, &c. 



The Arenig series was first recognized in North Wales by Prof. 

 Sedgwick abont the year 1843, and was then discussed by him in 

 papers presented to the Society. The Llandeilo series was discovered 

 by Sir R. Murchison previously in the Llandeilo district ; but its posi- 

 tion in the succession was not made out until about 1844. The Geo- 

 logical Survey have invariably included the Arenig in the Llandeilo 

 group ; but it was now shown that this occurred entirely from a 

 mistaken idea as to the relative position of the two series, which 

 were now shown to be entirely distinct groups, the equivalents of 

 both groups being present in Carnarvonshire, Shropshire, andTem- 

 brokeshire, but the Llandeilo group only of the two being developed 

 in Carmarthenshire. 



The lines of division in the series were said to be strongest at the 

 top of the Menevian group and at the top of the Tremadoc group, these 

 lines being palaeontological breaks only, and not the result of un- 

 conformities in the strata. 



December 16, 1874. — John Evans, Esq., E.R.S., President, 

 in the Chair. 



The following communications were read : — 



1. " Descriptions of the Graptolites of the Arenig and Llandeilo 

 Rocks of St. David's." By John Hopkinson, Esq., E.G.S., and 

 Charles Lapworth, Esq., E.G.S. 



Commencing wijiji a brief historical account of the " discovery of 

 Graptolites in the neighbourhood of St. David's, from their first 

 discovery in the Llandeilo series in 1841 by Sir Henry De la Beche 

 and Professor Ramsay, the authors proceeded to explain their view3 

 on the classification of the Graptolites (Graptolithixa, Bronn), 

 which they place under the order Hydroida, dividing them into two. 

 groups : — RnABDornoRA (Allman), comprising the true siculate or 

 virgulate Graptolites, which they consider to have been free or- 

 ganisms ; and Cladophoka (Hopkinson), comprising the dendroid 

 Graptolites and their allies, which were almost certainly fixed, and 

 are most nearly allied to the recent TJiecaphora. 



The distribution of the genera and species in the Arenig and 

 Llandeilo rocks of St. David's was then treated of; and the different 

 assemblages of species in each of their subdivisions were compared 

 with those of other areas. 



The Arenig rocks are seen to contain a number of species which 

 ally them more closely to the Quebec group of Canada than to any 

 other series of rocks, all their subdivisions containing Quebec 

 species ; while the Skiddaw Slates, which before the discovery of 

 Graptolites in the Lower Arenig rocks of Ramsey Island in 1872 



