THE AKENIG AND LLANDEILO ROCKS OP ST. DAVID'S. 637 



The only other district in Britain in which similar Graptolites are 

 known to occnr is near the Arenig Hills in Merionethshire, from 

 which this formation takes its name ; and although Graptolites and 

 other fossils had been collected in the Arenig area to the north-west 

 of these hills many years ago by Professor Sedgwick and Mr. Salter, 

 it is only in the present year that the equivalency of the containing 

 beds with the strata we are now considering has been conclusively 

 proved. 



Species of Bhabdophora characteristic of the Middle Arenig rocks 

 of St. David's also occur in the alum-slates of Sweden, in the lower 

 zones of the Oslo Group of Norway, in Russia, and in Newfoundland ; 

 and the valuable paper of Mr. B. Etheridge, jun., on the Grapto- 

 lites of Victoria*, enables us to recognize them as present even in the 

 antipodal Graptolitic rocks of the continent of Australia. 



Of the well-marked group of Ehabdophora furnished by the 

 Upper Arenig rocks of Llanvirn and Bamsey Island, most of the 

 species, including Dldymograptus patulus, D. blfidus, D. indentus, 

 D. Nicholsoni, Climacograptus confertus, and forms intimately allied 

 to Glossograptus ciliatus and Nemagraptus capillaris are found 

 in the Upper Skiddaw Slate series of Professor Nicholson, the same 

 species occurring in Wales and in Cumberland in almost exactly the 

 same relative abundance. Similar Graptolites are yielded by the 

 shales lying immediately below, and by those interstratified with, the 

 earliest trappean beds surmounting the great mass of the so-called 

 Lower Llandeilo rocks (Arenigs) of the neighbourhood of Shelve, 

 Shropshire. Dldymograptus patidus, D. bifidus, D. Nicholsoni, and 

 Diplograptus dentatus are here also the predominating forms. 



It is a noteworthy circumstance that three of the most peculiar 

 fossils of this Upper Arenig subformation, viz. Nemagraptus capil- 

 laris, Glossograptus ciliatus, and Diplograptus dentatus, are found in 

 the Taconic Shales of Dr. Emmons, a subdivision of that enigmatical 

 series of rocks which runs through the eastern portion of the State 

 of New York parallel with the course of the Hudson Biver. 



The Graptolites afforded by the well-known black shales of the 

 true Lower Llandeilo subformation as exposed in Abereiddy Bay, 

 form collectively a much larger group than those yet furnished by 

 equivalent deposits elsewhere in Britain. The four species of Cla- 

 dophora are as yet wholly restricted to this locality ; but many of the 

 Ehabdophora have a wide geographical range. 



The most characteristic Graptolite of the Lower Llandeilo beds 

 {Dldymograptus Murclilsonl) occurs in abundance in dark shales 

 associated with and surmounting the higher portion of the trappean 

 group of Shelve — beds which admit of almost exact parallelism with 

 the Lower Llandeilos of Abereiddy Bay on many other grounds. 

 The same species is also found in the strata which imbed the fel- 

 spathic traps and ashes of the neighbourhood of Builth and Llandrin- 

 dod. Here also it is associated with Diplograptus follaceus, D. trlcor- 

 nis, and Climacograptus ccelatus, its constant companions in the 

 lowest zone at Abereiddy. 



* Ann. & Mag. Nat. Hist, July 1874. 



