Vol. 65.] SCUCESSION AROUND PLYNLIMON AND PONT °EItWYD. 501 



The shaly band No. 2 forms the west side of the quarry (F. 37), 

 and is much weathered and broken up into thin slabs ; the chief 

 forms collected from it were : — Rastrites approximates, R. capil- 

 lars, R. hyhridus, Monograptus ansulosns, M. convolutus (v.c), ilf. 

 tenuis (?), M. harpago, M. limatulus, M. lobiferus (c), M. regularls, 

 Glyptograptus tamariscns , Climacograptus hug/iesi, and CI. sca- 

 laris (?). 



This assemblage of species indicates very clearly a high horizon 

 in the zone of Monograptus convolutus. The remarkable band 

 No. 4, which forms the east side of the quarry (F. 38), yielded in 

 the lower part a large number of species in beautiful preservation 

 in full and half relief. Owing probably to the local coincidence 

 of the cleavage with the bedding, the thin band of shales in which 

 the fossils occur can be removed in large slabs, one or both surfaces 

 of which not uncommonly carry half-a-dozen or more species of 

 graptolites. Such slabs, however, are not so easily procurable now 

 as formerly, for the fossiliferous band is only a few inches thick, 

 and a very large number of specimens have been collected from it. 

 The assemblage of species from it is very characteristic : — Retiolites 

 (Gladiograptus) periatus, Rastrites approximates var., R. hyhridus 

 (v.c), R. phleoides, Monograptus ansidosus, M. clingani, M. con- 

 volutus var. (v.c), M. decipiens, M. harpago, M. cf. involutes (a), 

 M. limatelus, M. lobiferus (c), M. cf. nudus, M. regxdaris (v.c), M. 

 tenuis (?), Glyptograptus tamariscus and var., Petalograptus minor 

 and cf. minor, P. pahneus and var. latus, Cephalograptus cometa, 

 Climacograptus hughesi, CI. scalaris, and other species not yet 

 determined. 



The presence of the restricted species Cephalograptus cometa in 

 this band is important, in allowing of its comparison with distant 

 areas. The ironstained shales which occur about 25 feet above 

 the band are lithologically similar to those of the Monogrctptus- 

 tenuis band, but from the limited amount of material available I 

 was not able to obtain that species. 



Although the beds in the quarry are vertical, there is no question 

 as to the band on the east side being higher than that on the west : 

 for, on the slope to the west of the quarry, the black shales of the 

 Monograptus- communis zone are exposed. 



(c) Slope south of Eisteddfa Gurig. — The only remaining 

 section east of Pont Erwyd to which I need refer is that along 

 the steep slope south of Eisteddfa Gurig. It forms the natural 

 continuation of Section III described on p. 478, wherein it was 

 mentioned that the black shale group was followed by pale mud- 

 stones and flags. In one place, at the base of these mudstones, a 

 few inches of a shale band are exposed, and yielded Monograptus 

 gregarius and M. cf . millipedia, the latter not having been observed 

 outside the leptotheca band, while the former occurs commonly in 

 that band. 



On account of the exposed position of the overlying rocks, they 



