Yol. 62.] TARATWOX SEKIES OF TARANNOE". 683 



main Twymyn River for a few yards, but soon become intermixed 

 with black shale-bands. Two of these exposed in the eastern bank 

 at point 4 in the plan, fig. 5, p. 680, yield : — 



Monograptus convolutus, His. (f. 0). 

 Monograptus gregarius, Lapw. (f. 0). 

 Monograptus communis mut. (f. C). 

 Monograptus leptotheca, Lapw. (R). 



Monograptus triangularis (?) His. 

 Climacograptus scalaris (0). 

 Diplograptus sinuatus (f . C). 

 Petalograptus minor, Elles (R). 



These beds dip steadily in a north-westerly direction under the 

 Twymyn Group, and must therefore be on a lower horizon. The 

 fauna is essentially distinct from that of the ' SedgivicJcii '-Beds, 

 for of eight species only two survive into the higher zone. They 

 should therefore be regarded as constituting part of a distinct group 

 of the Llandovery Series, which may be termed the Dolgadfan 

 Group, from the neighbouring hamlet. The characteristic grap- 

 tolite of its fauna is Monograptus convolutus, which has consequently 

 been selected as the local zone-fossil. 



Continuing the section down the valley, we find the Twymyn 

 turning sharply to the east, and along this east-and-west reach of 

 the river the Llandovery rocks present much the same lithological 

 character of finely-banded grey and black shales and mudstones. 

 They are, however, much folded and fractured and penetrated by 

 quartz-veins, although the general dip is 70° to 90° north- 

 westward. Such few graptolites as were obtained were clearly 

 of the Dolgadfan type. 



Leaving this part of the river, and going farther down to the 

 point 5 (see fig. 5, p. 680) where the river flows westward, we find 

 exhibited along the northern bank the nose of an anticlinal fold, 

 much faulted. Almost in the centre of this fold a thin black shale- 

 band, splitting into layers no thicker than a sheet of paper, yields 

 an abundance of well-preserved graptolites. The following species 

 are present : — 



Monograptus leptotheca (v. C). 

 Monograptus cf. decipiens, Tornq.(v.C). 

 Monograptus gregarius (C). 

 Monograptus harpago, Tornq. (f. C). 

 Monograptus convolutus (R). 

 Monograptus nuntius, JBarr. (R). 

 Climacograptus Htighesi, Nich. (=un- 



dulatus, Knrck.) (C). 

 Climacograptus scalaris. 



Petalograptus palmeus. 



Petalograptus ovato-elongatus, Kurck. 



(f.O). 

 Petalograptus minor, Elles. 

 Cephalograptus petalum, Elles. 

 Diplograptus magnus, H. Lapw. (f. C). 

 Diplograptus bellulus, Tornq. 

 Diplograptus sinuatus (?) . 



This fauna, though much richer in species, has considerable 

 similarity to that of the zone of Monograptus convolutus already 

 described, and may be referred to the same zone ; but it is suffi- 

 ciently distinct to be regarded as marking another, and probably 

 lower, band in that zone. 



Immediately to the east of this anticlinal axis, and faulted against 

 the Dolgadfan Beds, occur thick black shales which both in litho- 

 logy and in fauna clearly belong to the overlying ' SedgwickiV- 

 Beds. They yield Monograptus SedgivicJcii and M. discretus. The 

 beds are much disturbed, but dip off the anticline at high angles 

 north-eastward. 



