134 Me Learn — Silurian Arisaig Series of Arisaio, .N. S. 



it was in western England and in Wales that the Silurian 

 was first studied and the Silurian system was established. 

 The Beechhill is correlated with the Lower Llandovery 

 of Wales, the Haverford stage of Haverfordwest in 

 southwest Wales and the Mulloch Hill and Saugh Hill 

 groups of Girvan in south Scotland. This is based in 

 part on negative evidence, in the absence of such char- 

 acteristic Upper Llandovery species as Plagiorhyncha 

 decemplicata (Sowerby), Ccelospira hemisplierica (Sow- 

 erby), and Dalmanites weaveri (Salter). More positive 

 evidence is found in the presence of a variety of the 

 typical Lower Llandovery, Whitfieldellaf crassa (Sow- 

 erby). Accumulative evidence is sought in the presence 

 of a single specimen of W.f angustifrons (McCoy) and 

 the close resemblance of the Beechhill Rhipidomella 

 (Schizophorella) arisaigensis, n. sp., to the Mulloch Hill 

 R. (S.) mullochensis (Davidson). The resemblance of 

 the Mulloch Hill Ccelospira scotica to C. planoconvexa 

 (Hall), of which at best it is likely not more than a vari- 

 ety, suggests that the British equivalent of the American 

 Cataract is to be found in the Mulloch Hill and probably 

 also the Saugh Hill group of Girvan. 



The Ross Brook correlates with the remainder of the 

 Valentian, i. e., the Upper Llandovery and Tarannon of 

 Wales and also the Mullin and Rosemarket stages of 

 Haverfordwest and the Camregan and Penkill groups of 

 Girvan. This is based chiefly on the highly diagnostic 

 species: Plagiorhyncha decemplicata (Sowerby), Ccelos- 

 pira hemisplierica (Sowerby), and Dalmanites weaveri 

 (Salter). For the first time the Tarannon is recognized 

 in intercontinental correlation and indeed in any correla- 

 tion founded on fossils other than graptolites. This is 

 based on the writer's interpretation of the recent work 

 of F. R. C. Reed (1917). The Tarannon has heretofore 

 been recognized only by its characteristic graptolite 

 faunas, other phyla being too poorly represented to have 

 any significance. In the Penkill group of Girvan both 

 graptolite and brachiopod phases are closely associated 

 and the former establishes its Tarannon age. Reed's 

 list of brachiopods includes such characteristic Llandov- 

 ery species as C. hemisplierica, Clorinda undata and 

 Pentamerus oblongus, the first of which does not range 

 below the Upper Llandovery. When correlating with 

 brachiopods, therefore, the Tarannon is best combined 



