430 



ME. A. WADE ON THE LLANDOYEKY AND 



[Aug. 



The Gwern-y-brain Series is not more than 50 feet thick. The 

 limestone-band is only a few feet thick, and is unfortunately 

 crystalline, yielding no fossils. It appears to correspond, in its 

 stratigraphical position, with the Staurocephalus Limestone at the 

 base of the Ashgillian. The oYerlying black shales are of the nature 

 of passage-beds — since they contain Trinucleus cf. seticornis, asso- 

 ciated with a peculiar assemblage of graptolites comprising forms 

 which suggest an horizon, transitional between the Upper Hartfell 

 and the Lower Birkhill, at which graptolites appear to be generally 

 scarce and not well known. 



The exact relation between these beds and the underlying grits 

 is obscured by strike-faulting, while the Powis-Castle Conglomerate 

 quickly overlaps them above. Obviously, it would be incon- 

 venient to draw the line between the Ordovician and the Silurian 

 in these shales ; and therefore, since the fauna seems to justify 

 this correlation, I have regarded them as being equivalent to the 

 Ashgillian of Dr. Marr. 



*-< rA 





*"< rA 







List o e Fossils 





PROM THE 





GwEEN-T-BEAIlf 





Shales. 



X 



Calymene blumenbachi Brongn. 





Conularia cf. aspersa Lindstrcem. 





(Ranges into overlying beds.) 





*Eccyliomphalus bucklandi Portlock. 





*Trinucleus cf. seticornis His. 





*Ecct/liomphalus minor Portlock. 





JBollia lata Vanux. & Hall. 





*l£ccyliomplialus contiguus var. cam- 





Ctenobolina ciliata Emmons. 





brensis, nov. (see p. 454). 





Ctenobolina cf. ciliata Emmons. 





Orbiculoidea perrugata M'Coy. 





*TLrausella arcuata Ulrich. 





Orbiculoidea cf. perrugata M'Coy. 





*Leperditia nana (?) Jones. 





Orbiculoidea cf. crassa Hall. 





*Primitia Jntmilis,vav.humilior Jones. 



X 



Lingula attenuata Sowerby. 





*Primitia morgani Jones. 





Lingula cf. brevis Portlock. 





*Primitia mundula, var. cambrica 





.Lingula obtusiformis , sp. nov. (see 





Jones. 





p. 455). 





*Primitia tumidula Ulrich. 





Orbicula cf. terminalis Conrad. 





*Primitia cf. ulriclii Jones. 





Obolella sp. 





*Primitiella unicornis Ulrich, et var. 





Orthis himantensis M'Coy. 





Ilelanella hemidiscus, gen. et sp. nov. 





Orthis sagittifera M'Co} r . 





(see p. 451). 





Orthis intercostata Portlock. 





*Corynoides calycularis Nicholson. 



X 



OrtMs (Dalmanella) elegantula 





*Dendrograptus sp. 





Dahn. (Ranges into overliving 





Climacograptus sp. 





beds.) 





^Diplograptus (3Iesograptus) mod- 



X 



OrtMs cf. valpyana "Davidson. 





estus, cf. var. parvulus H. Lapw. 



X 



PI ectambonites sericea~) Range into 





Diplograptus (Ortlwgraptus) trun-\ 





Sowerby. > overlying 





catus, cf. var. socialis Lap worth. 





PI. transversalis Dalm.J beds. 





Diplograptus sp. 





Trematis corona Salter ? 



::: 



Glyptograptus persculptus Salter. 





Siphonotreta micula M'Coy. 



* From J. Bickerton Morgan's Collection. 



It will be noted that out of 42 species only five range upwards 

 from the beds below ; four of these pass upwards also, being forms 

 notable for their width of range. The fossil list thus strongly 

 confirms the evidence afforded by the lithological character of 

 the beds. It is evident that a remarkable change took place 

 between the periods of shallow-water deposition during which 



