The Carboniferous Succession heloio the Coal-Measiires. 499 



In the Bristol area Prodiictiis giganteus with its varietal forms has 

 its maximum towards the base of the Lower Dibunophyllnm sub-zone, 

 whereas in North Wales this group of shells is not at all common at 

 this horizon, but occurs in the greatest profusion at the top of the 

 Upper Dihunoplujllum sub-zone. At the quarry at Waenbrodlas, 

 immediately below the Cyathaxonia beds, this species occurs in 

 millions, and curiously enough the named varietal forms of this 

 shell occur with it in the same beds. It is apparent, therefore, 

 that the evolution of varietal forms of this species does not afford 

 any help for the purposes of zoning. 



The typical transverse forms of P. giganteus, as figured by Martin, 

 with long ear-like processes aiid fine ribs, are accompanied by the 

 broad flat-ribbed form named P. Edelhurgensis by Phillips, and 

 others less transverse like the specimens figured by Davidson (Brit. 

 Foss, Brachiopoda, pt. v, pi. xxxvii, figs. 1-3). 



The Lower Dibimophyllum zone is marked by the presence of 

 Gyathopliyllum Murchisoni in abundance. Lilhostrotion Martini and 

 its mutations are also fairly abundant. Dibunophyllum and J). are 

 also present. 



Productiis hemisphericus and P. Cora are also frequent. 



North Wahs. 



BRISTOL , 



Fig. 4. — Diagrammatic correlation of the Carboniferous succeesion in the Bristol 

 district and in North "Wales. 



The Upper Dibunophyllum sub-zone is characterised by all the 

 typical fossils of the Bristol equivalent : Lonsdaleia floriformis, 

 L.rugosa, Cyathophylhim of. regium, C.3IiircJiisoni, Zithostrotion Martini, 

 L. irregidare, L. Junceum, L. Portlochi, L. ensifer, Alveolites septosa, 

 Dibunophyllum turbinatum, and Phillipsastrcea radiata. The fauna is, 

 however, much richer in genera, species, and individuals tiian that 

 which is present in the Upper Dibunophyllum zone of the Bristol 

 area. Spirifer and its allies are abundant: S. trigonalis, S. bisul- 

 catus, S. planicostata, S. striata, Beticularia lineata, Martinia glabra, 

 Athyris planosidcata, A. expama, Seminula ambigua, Dielasma 

 hastata, and various forms of Pugnax and Camarotcechia. 



Schizophora resupinata is very abundant and of large size, but 

 Syringothyris cuspidata is decidedly rare. 



It is in this sub-zone that the well-known rich Carboniferous 



