682 MISS E. M. E. WOOD ON THE [NOV. 1906, 



yields an abundance of graptolites. These are generally preserved 

 as white films ; and, while the identification of some of the more 

 abundant forms with known species was a matter of difficulty, 

 yet it was possible to recognize : — 



Monograptus distans, Portl. (v. 0). 

 Monograptus jaculum, Lapw. (C). 

 Monograptus discretus, Nich. (0). 



Monograptus runcinatus (E). 

 Monograptus galaensis mut. (f. C). 1 

 Dipiograptus tamariscus, Nich. 

 Climacograptus scalaris, His. (?). 



Monograptus Clingani, Carr. (E). 

 Monograptus Sedgwickii, Portl. (v. C). 



Still lower down, at the point where the footpath from Llanbryn- 

 mair village to Bont Dolgadfan crosses the stream, is an exposure of 

 intensely-black soft shale, dipping north-westward at an angle of 57° 

 to 60° (3 in fig. 5, p. 680). This shale is carbonaceous, and blackens 

 the fingers. It weathers to a deep orange tint, and has a curious 

 conchoidal fracture. An extensive dip-surface is exposed, and on 

 this the graptolites, which are preserved as whitish films, show out 

 conspicuously. The following species are recognizable : — 



Diplograjrtus sinuatus, Nich. (f. C). 

 Dipiograptus tamariscus. 

 Climacograptus scalaris (f. C). 



Monograptus Sedgwickii (v. C). 

 Monograptus discretus (=M. tenuis, 



Tornq.) (v. C). 

 Monograptus distans (?). 



This black-shale band has a thickness of about 4 feet, but is not 

 equally fossiliferous throughout. It crops out again on the south 

 side of the footpath, and in the bed of the stream below the foot- 

 bridge ; and, between that point and the Twymyn River, the rocks 

 laid bare consist of light-coloured brownish-grey shales and mud- 

 stones containing patches of carbonaceous matter, but with no 

 distinct black layers. 



Thus, rising out from below the Brynmair Beds we find an 

 uninterrupted succession of about 140 feet of grey shales and mud- 

 stones, with at least two black shale-bands. The faunas of these 

 bands have several species in common, and may be referred to one 

 zone which, on account of the great abundance of M. Sedywickii, 

 may be appropriately called the local zone of Monograptus 

 Sedgivickii. Here, for the first time in the descending order 

 of beds in this district, do we find a graptolitic fauna which is 

 acknowledged to be of Upper Llandovery age — namely, that of the 

 Upper Birkhill Shales of Southern Scotland and of the Caban Group 

 of Rhayader ; and we have, therefore, descended below the base-limit 

 of the Tarannon Series. I term these beds the Twymyn Group; 

 and they constitute the highest member of the Llandovery Series 

 of this district, as distinct from the overlying Tarannon Series. 

 The Twymyn Group may be here divided into : — (1) M.-SedgivicMi 

 Beds (Ac 2 ), with two fossiliferous bands (90 feet); and (2) pale- 

 grey mudstones (Ac x ) (50 feet), containing no fossils at this locality. 



Dolgadfan Group (Ab). (Zone of Monograptus convoluius.) — 

 The Lower Twymyn pale mudstones are continued in the bed of the 



1 The form here named provisionally ' Monograptus galaensis mut.' occurs 

 in the M.-Sedgvjickii Beds of the South of Scotland, and is probably a new- 

 variety. 



