ON AEENIG AND LLANDEILO GKAPTOLITES FKOM ST. "DAVID'S. 631 



47. Descriptions of the Gkaptolites of the Akenig- and Llandeilo 

 Bocks of St. David's. By John Hopetnson, Esq., F.L.S., F.G.S., 

 and Chaeles Lapworth, Esq., E.G.S. (Read December 16, 

 1874.) 



[Plates XXXIII.-XXXVIL] 



Inteodtjction. 



In the neighbourhood of St. David's, the Arenig rocks, described by 

 Mr. Hicks in his paper read at the last meeting of the Geolo- 

 gical Society*, are the earliest in which Graptolites are known to 

 occur ; and yet, when once they appear, so diversified are their forms 

 that these Pembrokeshire rocks are only equalled, in the number 

 and variety of the genera they contain, by the Canadian Graptolite- 

 bearing rocks of equivalent age known as the Quebec Group. In 

 more ancient deposits two species only, belonging to one of the two 

 great sections into which these fossils are divided, have hitherto 

 been detected, viz. Dictyograptus (Dictyonema) socialis, Salter, and 

 Dendrograptus Hallianus, Prout. The former occurring in the 

 lower portion of the Tremadoc rocks of Worth Wales, and the latter 

 in the equivalent strata (the Potsdam Sandstone of America), it is 

 impossible to say which genus is the earlier, or whether the group 

 is first represented in Britain or in America. 



Before the discovery, in 1872, of the extensive series of Graptolites 

 which characterize the Lower Arenig rocks of Eamsey Island, the 

 Skiddaw Slates of Cumberland were supposed to be our earliest 

 Graptolite -bearing rocks ; but it is now known that the lowest rocks 

 of the Arenig Group exposed in the vicinity of St. David's, in which 

 Graptolites abound, are of greater age than any part of the Skiddaw 

 Slates yet described ; and it is highly probable that they are also 

 older even than the lowest beds of the Quebec Group known to con- 

 tain Graptolites, as will presently be shown. 



As early as 1841 Graptolites were discovered in the " black slaty 

 rocks of Pembrokeshire " (in the Llandeilo series) by Sir Henry De 

 La Beche ; and soon afterwards Professor Ramsay found the well- 

 known species Didymograptus Murchisoni in the Llandeilo rocks of 

 Abereiddy Bay ; but it is not until 1866 that we find the first men- 

 tion of the occurrence of these fossils in the older rocks of this district. 

 In their 'Second Beport' on the St. David's rocks, presented to the 

 British Association in that year (1866), Messrs. Salter and Hicks 

 mention the occurrence of the genera Didymograptus andDendrograp- 

 tus in the black slates and flags of Whitesand Bay, which they then 

 recognized as being of Arenig age ; and in the same year, in a paper 

 by Mr. Wyatt Edgell, " On the Arenig and Llandeilo Groups " f , the 



* " On the Succession of the Ancient Eocks in the vicinity of St. David's, 

 Pembrokeshire, with special reference to those of the Arenig and Llandeilo 

 Groups, and their Fossil Contents." Quart. Journ. Geol. Soc. vol. xxxi. p. 167. 



t Proc. Geologists' Assoc. July 1866. Geological Magazine, vol. iv. p. 113. 



