part 2] 



ITS STEUCTURE AND EOCK-SrCCESSION. 



145 



likely that the graptolites Dicranograptus rectus Hopkinson and 

 Amplexograptus perexcavatus Lapworth, found by Prof. Fearn- 

 sides in a loose block of shale on Mynydd Nodol, may have come 

 from the same beds, since they, like the others, are characteristic 

 fossils of the Upper Grienkiln Shales. The Derfel Limestone 

 would, therefore, appear to be of Llandilian age ; and, indeed, its 

 fauna is a very remarkable one, since it is not of the type usually 

 characteristic of the Welsh Llandilian, but approximates more 

 closely to the fauna of the Scottish deposits of that age — more 

 especially, perhaps, that of the Stinchar Limestone. The following 

 are the more important forms : — 



Lichas laxatus M'Coy. 

 Cyhele verrucosa (Dalman). 

 Cyhele rugosa (Portlock). 

 Illaenus cf. halclatchiensis Reed. 

 Acidas^ris hysterix Wyville Thomson. 

 Harpes flanagani Portlock. 

 Trinucleus sp. 



Orthis (PlatystrophiaJ hiforata 



(Schlotheim). 

 Orthis fNicolellaJ actonise (Sowerby). 

 Orthis (HebertellaJ crispa (M'Coy). 

 Orthis alata Sowerby. 

 Orthis (DalmanellaJ testiidinaria 



var. gracilis Reed. 

 Orthis fDalmanellaJ girvanensis 



(Davidson). 

 Orthis playfairi Reed. 



Orthis (HeterorthisJ co?i/ims (Salter?). 



Skenidium leivisi var. craigense 

 Reed. 



Triplecia craigensis Reed. 



Clytonia andersoni Reed. 



Clitamhonites asceridens Pander ? 



Leptsena rhomhoidalis Wilckens. 



Rafinesquina expansa var. 



Rafinesquina suharachnoidea 

 Reed(?). 



Stropheodonta corrugatella (David- 

 son). 



PlectamhonitesllandeiloensisiDa^Yid- 

 son). 



Plectambonites sericea var. serni- 

 rugata Reed. 



Numerous Bryozoa and a few Cri- 

 noid-stems and Cystid-plates. 



Nant Rhyd Wen. 



Nant Rhyd Wen affords the best section in the Grlyn-Gower 

 Beds, up to their junction with the Frondderw Ash. The massively- 

 bedded Grlyn-Grower Sandstones are seen close to the junction of 

 the stream with the Afon Glyn dipping 30° south-eastwards, and 

 these occupy the bed of the stream for about 140 yards ; then some 

 soft shale-bands are occasionally seen alternating with the sand- 

 stones : one of the lowest yielded traces of graptolites (^Diplograpti)^ 

 including Orthograptus truncatus Lapworth, and other bands 

 yielded Trinucleus of the concentricus type, Glyptocrinus 

 hasalls, and Beyrichia. These beds, which are now more sandy 

 mudstones than sandstones, continue for another 300 yards until 

 the stream makes a definite bend, and, instead of running north- 

 westwards, turns nearly due north. At this point a low clifE on 

 the right bank shows a section of some interest : at the top of the 

 clifE the Frondderw Ash is seen, and at the bottom a good crystal- 

 line limestone, a few feet of sandy mudstone separating the two. 

 The limestone appears to be merely a lenticle, for it dies away 

 rapidly northwards and southwards, and is never more than 

 18 inches thick ; it is, however, richly fossiliferous, the following 



Q. J. O. S. No. 310. L 



