Vol. 62.] TAEANNOjNT SERIES OP TARANNOtf. 681 



mudstone-band. They are too fragmentary and distorted to be 

 identified with accuracy ; but it is possible to recognize Monograptus 

 turriculatus (v. R), M. cf. galaensis, Lapw. (C), M. Halli? (f. C), 

 M . proteus (? ) , and Retiolites sp. 



This fauna is very scanty and indefinite, but it would appear to 

 belong to the Brynmair Group, although it occupies a distinctly-lower 

 horizon than that of the beds at Bont-y-Green. That there is here 

 an anticlinal fold, accompanied by a fracture of some importance, 

 is more clearly seen at the next bend of the river farther north, 

 where the right bank is occupied by a thick mass of quartz in the 

 neighbourhood of which the beds are much crumpled ; while every- 

 where to the east of this quartz-vein the beds dip south-eastward. 



(6) Sections showing the Eelation of the Tarannon 

 Series to the Llandovery Series in the Twymyn Valley. 



(i) Pontbren-dibyn and the Twymyn near Dolgadfan. 



The stratigraphical relation of the Tarannon to the underlying 

 Llandovery Series is well exhibited in three sections of the 

 Twymyn ; the northernmost, and perhaps the most interesting, of 

 these is that in the valley of the Twymyn near Bont Dolgadfan, 

 and in the small tributary stream of the Pontbren-dibyn which 

 joins it on its left bank. (See fig. 5, p. 680.) 



Tarannon Series. 



Brynmair Beds (Ba). (Zone of Monograjptus turriculatus.) — 

 The Pontbren-dibyn flows for the lower 300 yards of its course 

 through a fir- plantation, about a quarter of a mile south of Llan- 

 brynmair village. At a point in the stream less than 100 yards 

 below the main road, and just above a small hedge on the left bank 

 (1 in fig. 5, p. 680), some grey- and black-banded mudstones are 

 exposed, in the darker layers of which the following graptolites 

 were found in a good state of preservation : — 



Rastrites maximus, Carr. (R). 

 Rastrites Linncei (C). 

 Monograplus turriculatus (f. O). 

 Monograptus nudus (.C). 

 Monograptus Becki (f. C). 



The beds at this locality dip 26° north-westward. The presence 

 of M. turriculatus in fair abundance fixes the age of these beds 

 as that of the Brynmair Group ; but it is interesting to find that, 

 in addition to the forms already obtained from them, there is the 

 well-known species Rastrites maximus. This has been found at 

 this one locality only. 



Llandovery Series. 



Twymyn Group (Ac). (Zone of Monograptus Sedgwickii, 

 Ac 2 .) — Proceeding down the stream for 50 to 60 yards, we find 

 interbedded with pale-grey shales and mudstones, and weathering 

 to an orange-brown tint, a soft black-shale band (2 in fig. 5), which 



3a2 



Monograptus subconicus ? (R). 

 Monograptus gemmatus, Barr. 

 Monograptus Halli ? (Barr.). 

 Petalograpitus palmeus mut. (f. C). 



