Vol. 65.] HARXFELL-VALENTIAN SUCCESSION ABOUND PLYNLIMON. 505 



section to draw a hard-and-fast line on lithological grounds between 

 these strata and the top of the sedgwicli zone. The transition 

 must take place somewhere in the neighbourhood of the Parson's 

 Bridge, and it is possible that the thin grits which make their 

 appearance just below the bridge mark the oncoming of the newer 

 group. 



It is unnecessary to describe in detail the other sections of the 

 Castell Group. It will suffice to refer briefly to the one on the 

 hillside south of Bryn-bras. The convolutus beds are somewhat 

 gritty in places, in this respect resembling those in the gorge south 

 of the Castell Fault. They are followed by 110 to 120 feet of smooth 

 blue flags, which yielded (F. 42) Monograptus clingani, M. convo- 

 lutus (?), M. crenularis var. a (Lapw.), M. jaculum, M. sedgwicki, 

 and Acanthograptus ramosvs (?) near the base ; while thin grits 

 appear in the upper part, forming a transition to the" group of pale 

 mudstones and shales with thin black-banded grits which furnished 

 the lurriculatus fauna, that form being found in considerable 

 numbers with Monograptus becki, M. runcinatus, and dendroid 

 graptolites. 



I need only refer to one other fossiliferous locality in the western 

 belt of the Pont Erwyd Beds, which runs northwards from the 

 village along the Rheidol Valley. This is found at the top of the old 

 quarry by the stream north-east of Gwenffrwd-uchaf Farm, where 

 the leptotheca band was mentioned on p. 492. That band is followed 

 by 43 feet of smooth, thickly-bedded, greenish-blue flags, which 

 were worked in the quarry ; they pass up gradually into a band of 

 deeply-weathering rusty shales (F. 43), which yielded the follow- 

 ing assemblage characteristic of the M. convolutus zone : — Rastrites 

 approocimatus, R. capillaris, R. liybridus (?), R. peregrinus, Mono- 

 graptus ansidosus, M. concilium (?), M. convolutus (v. c), M. decipiens, 

 M. harpago (?), M. cf. involutus, M. limatulus, M. lobiferus, M. cf. 

 nudus, M. regularis, M. tenuis (?), Glyptograptus tamariscus, 

 Glimacograptus hughesi, and CI. scalaris. 



This band forms the top of the quarry, and is followed by pale 

 greenish shales and mudstones with thin black-banded grits ; all 

 the beds weather to a pale lemon-colour, and are not unlike the 

 M.-turricidatus beds. A thickness of 80 to 100 feet is exposed ; 

 but I could detect no shale-bands among them, and I am unable to 

 say to what part of the succession they pertain. 



On p. 504 is a vertical section through the Pont Erwyd Stage, 

 showing the distribution of the graptolitic horizons and the 

 lithological character of the strata (fig. 11). 



The Fauna of the Pont Erwyd Stage. 



The fauna of the Pont Erwyd Stage consists almost entirely of 

 graptolites of the families Diplograptidas and Monograptidae. 



The Eisteddfa Group at the base contains Diplograptidae ex- 

 clusively ; the species Mesograptus mod est as and its varieties, as also 



