232 



Murchison s Silurian System. 



Terebratula furcata, 



unguis^ 



Terebratula neglecta, 



■ tripartita, 



decemplicata, 



pusilla, 



Tentaculites scalaris^ 



— annulatus. 



Llandeilo Flags. — The lower strata of Silurian rocks con- 

 sist of flags, named by Mr. Murchison like the other forma- 

 tions from the place in which he found the strata best deve- 

 loped. They consist of hard, dark-coloured flags, sometimes 

 slightly micaceous, frequently calcareous, with veins of 

 white crystallized carbonate of lime. These beds are, 

 however, better distinguished as the repository of large Tri- 

 lobites, called Asaphus Buchii, Bron. and A. tyrannus, Mur- 

 chison. These flags pass under ridges equivalent to the 

 Carodac, and are seen in some situations to graduate down- 

 wards into the older strata of the Cambrian or primary 

 rocks, the upper portion of which consists of overlying 

 masses, containing a few calcareous courses, with casts of 

 fossils 3 and at intervals thick and massive bands of prophy- 

 ritic trap, as well as thinly laminated yellowish green bands 

 of compact felspar, and other rocks of igneous origin. Among 

 the fossils found in these beds, Mr. Murchison enumerates 

 the following : — 



Orthis anomala, 



actoniae, 



canales, 



compressa, 



flabellulum ; 



lata, 



— pecten, 



protensa, 



testudinaria, 



