114 Wyatt-Bdgell — The Aremg and Llandeild Groups. 



from which, it is totally distinct, although classed with them in 

 Murchison's " Siluria." The name is derived from Arenig-fawr, 

 Merionethshire, where the group is well shown and full of igneous 

 rocks : from this locality Prof. Sedgwick and Mr. Salter obtained 

 two of the characteristic fossils, Calymene parvifrons and Ogygia 

 Selwynii. He recognized the beds as different from those above, and 

 called them Arenig slates, considering them the top of his Festiniog 

 group. (See " Introduction to Synopsis of Woodwardian Museum," 

 London and Cambridge, 1855.) But the group was not well known 

 as a distinct one, until the appearance of the last edition of Ly ell's 

 " Manual of Geology ;" since then it has been established more com- 

 pletely by the lists of fossils given in vol. iii. of the " Memoirs of 

 the Geological Survey." In England it is seen in the lead-mining 

 district of the Stiperstones, Shropshire, underlying the Llandeilo 

 ilags of Shelve and Gornden Hill ; here it is that Murchison more 

 especially describes it under the name of Lower Llandeilo. It 

 occurs again near St. David's Head, overlying the Cambrian, and 

 passing upwards into the Llandeilo flags ; here it was first recog- 

 nized by Mr. Salter, who frequently cites this locality in his " Mono- 

 graph of British Trilobites," now coming out. (See Palseontogr. 

 Monogr.) 



The distinctness of our group from the latter is evident when the 

 fossils of each are compared. The character of the fauna varies as 

 much as it does between the Llandeilo and the Caradoc — more so 

 than between the Caradoc and the Lower Llandovery, This will at 

 once be seen from the lists here given. 



A-R^mG.— Ogygia peltata; 0. Selwynii; Calymene parvifrons ; 

 jEglina hinodosa ; jE. grandis ; jE. caliginosa ; Trinucleus MurcM- 

 soni ; T. Gihbsii ; Orthoceras Avelinei ; Obolella plumhea ; Orthis sp. 

 (Whitesand Bay) ; Cucullella Anglica ; Encrinites (two). 



Llandeilo, — Lower. — Asajjfius tyr annus ; A. peltastes ; Ogygia 

 Bucliii var. convexa ; Calymene Cambrensis ; Trinucleus favus ; Lichas 

 patriarcJms ; Bellerophon hilohatus ; Eelicotoma sp. ; Lingula granu- 

 lata ; L. attenuata ; Orthis striatula ; Ctenodonta sp. 



Llandeilo, — Upper. — Asaphns Corndensis ; 0. BvcM, ordinary 

 var.; Barrandia {Ogygia) radians : B. Cordai ; Calymene duplicata ; 

 Trinucleus fimbriatus ; Cheirurus Sedgwicldi; Ampyx nudits ; Agnostus 

 Maccoyi; Lingula Bamsaiji ; B. perturbatus ; MurcUsonia simplex; 

 Modiolopsis injlata ; Didymograpsus MurcMsoni. 



The first fos.sil cited from the Arenig is from "Whitesand Bay, 

 near St. David's Head. It has recently been described by Mr. 

 Salter under the name of Ogtjgia peltata; this description will _ be 

 publislied, I believe, in the next volume of the Palajontographical 

 Society.^ . . 



Ogygia Sehotjnii, the next on the list, is the most characteristic 

 fossil of the Arenig, or Skiddaw, rocks. It is found both in the 

 Stiperstones district and in North Wales, and serves to mark these 

 beds, just as Ogygia BucM does the Llandeilo flags. Associated with 



1 This paper was written in March, 18G6. The volume referred to, has since been 

 published. 



