404 Sir Roderick L Murchisonh 



discoveries of organic remains by MM. Homfray and Ashe of 

 Tremadoc. 



" It has been thus ascertained, that the lower member only of 

 the deposit which has hitherto passed under the name of Lin- 

 gula flags, can be considered the equivalent of the primordial 

 zone of Bohemia. In North Wales that zone has hitherto been 

 mainly characterized by Lingula and the crustaceans Oleuus and 

 Paradoxides. Certain additions having; been made to these fos- 

 sils, Mr. Salter finds that of the whole there are five genera pecu- 

 liar to the lower zone, and seven which pass upwards from it into 

 the next overlying band, or the Tremadoc slate. But the over- 

 lying Tremadoc slate, hitherto also grouped with the Lingula 

 flags, is, through its numerous fossils (many of them of recent 

 discovery), demonstrated to constitute a true lower member of 

 the Llandeilo formation. For among the trilobites, the well- 

 known Llandeilo forms of Asaphus and Ogygia range upwards 

 from the very base of these slates. Again, seven or eight other 

 genera of trilobites, which appear here for the first time, are asso- 

 ciated with genera of mollusks and encrinites which have lived 

 through the whole Silurian series. Such, for example, are the 

 genera Calymene, Illsenus, among crustaceans ; the Lingula, Or- 

 this, Bftllerophon, and Conularia, among mollusks ; together with 

 encrinites, corals, and that telling Silurian, zoophyte, the Grapto- 

 lite. By this proof of the community of fossil types, as well as 

 by a clear lithological passage of the beds, these Tremadoc 

 slates are thus shown to be indissolubly connected with the 

 Llandeilo and other Silurian formations above them ; while, al- 

 though they also pass down conformably into the zone pri- 

 mordlale, the latter is characterized by the linguloid shells (Lin- 

 guella, Salter) and by the genera Olenus, Paradoxides, and 

 Dikelocephalus, which most characterize it in Britain as in other 

 regions.* 



" I take this opportunity, however, of reiterating the opinions I 

 have expressed in my work Siluria, that to whatever extent the 

 primordial zone of Barrande be distinguished by peculiar fossils 

 in any given tract, from the prevalent Lower Silurian types, there 

 exists no valid ground for diff"ering from Barrande, De Verneuil, Lo- 



* In the last edition of Siluria the distinction was drawn between 

 the lower and upper Lingula flags, but the fauna of the latter is now 

 much enlarged. 



