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PROF. W. ,T. PUUH ON THE GEOLOGY OF THE [vol. lxxix, 



extremely abundant, some of the bedding-planes being covered with 

 Dicellograpti and Climacograpti. These graptolites are, however, 

 commonly distorted, and not very well preserved. It is a curious 

 fact that such a characteristic band should have escaped the 

 attention of earlier investigators in the area. 



Graptolites are most easily obtained from the Nant yr aur and 

 Craig Hengae exposures. From the former I have collected fairly 

 well-preserved specimens of : — 



Dicellograptus forchammeri Geinitz. 



Dicellograptus morrisi Hopkinson. 



D'icranograptus clingani Carruthers. 



Climacogra/ptus minimus (Carruthers). 



Climacograptus scalaris var. misera- 

 bilis Biles & Wood. 



Climacograptus styloideus Lap- 

 worth. 



Climacograptus tubuliferus Lap- 

 worth. 



Orthograptus calcaratus var. basili- 

 cus Lapworth. 



Orthograptus truncatus var. socialis 

 Lapworth. 



Orthograptus sp. 



Glyptograptus (?). 



Mr. G. J. Williams informs me that he has found Plegmato- 

 grapfus nebula Elles & Wood in this locality. 

 From Craiff Hengae I have obtained : — 



Dicranograptus clingani Carruthers. 

 Dicranograptus cf. clingani Car- 

 ruthers. 



Climacograptus styloideus Lap- 

 worth. 



Orthograptus cf. quadrimucronatus 

 Hail. 



In addition to these graptolites, I have collected from exposures 

 a short distance north-east of the area at present described 

 Orthograptus quadrimucronatus (Hall) and O. truncatus var. 

 pauperatus Elles & Wood. 



The most abundant graptolites are the Dicellograpti and the 

 Climacograpti, particularly Climacograptus minimus. Specimens 

 of Dicranograptus are relatively rare, and, although they belong 

 in all probability to the species D. clingani, they are small and 

 dwarfed forms. 



The general aspect of the fauna seems to indicate that of the 

 zone of Dicranograptus clingani ; but there are several forms 

 which suggest also the higher horizon of Dleurograptus linearis. 

 The specimens of Dicellograptus appear to occur most abundantly 

 towards the top of the black shales. At a slightly lower horizon 

 the bedding planes may be crowded with Climacoarapti : for in- 

 stance, Climacograptus minimus; while the black shales which 

 contain Dicranograptus are probably at a somewhat lower horizon 

 still. It is possible that there are different levels within this black 

 shale-band, which may be characterized by somewhat different 

 assemblages of graptolites, although, so far, I have not been able 

 to investigate this in detail. The rocks are highly cleaved, and, in 

 many cases, it is difficult to extract recognizable forms. 



It is possible, therefore, that the Nod Glas may actually represent 

 the junction between the zones of Dicranograptus clingani and 

 Pleurograptus linearis, or indeed that the very highest layers of 

 the black shale may belong to the latter zone, 



