Vol. 56.] PORMATION AND ITS GRA.PTO LITE-FAUNA. 



435 





As I have already mentioned, these upper flags and mudstones 

 are rich in brachiopods, especially certain calcareous bands. One 

 bed composed largely of casts of Dayia navicula is well seen in 

 a quarry at Erw-gilfach, about 1 mile south of Builth, in association 

 with Spirifera crispa and Scenidium Lewisi (kindly identified by 

 Mr. F. R. C. Eeed). The first-named brachiopod occurs in great 

 abundance in the higher beds of the Lower Ludlow, in the area 

 lying east of the Wye, and does not seem to be confined to any one 

 horizon. I have not traced it for any distance in the south-western 

 area. 



(c) Summary. — Reviewing the whole of the evidence obtained 

 from this area, the Lower Ludlow formation would seem to be 

 divisible into three groups : — 



(3) Mudstones and calcareous flags. Zone of Monograptus leintwardinensis. 

 (2) Shales and thin beds of limestone rich in graptolites. Zone of 



Monograptus Mlssoni. 500 to 600 feet. 

 (1) Hard flags. Zone of Monograptus vulgaris. 100 feet. 



Fig. 6. 











V •' .-> /• a/401 



/ 



SJke£ck,JWah/. ,J 

 eflke> ^ 

 Builth District 

 \&pm -'J?™ (feasijireaA 



(4) Area lying East of the Wye. (See map, fig. 6.) 



Coming now to the area of Lower Ludlow Shales which lies 

 east of the Wye and occupies the slopes of the Aberedw Hills, 

 one finds that the beds present certain differences from the Builth 

 area proper. The richly fossiliferous band of the Irfon, etc. is but 

 poorly represented ; while, on the other hand, the upper horizons, 



