108 PROCEEDINGS OE THE GEOLOGICAL SOCIETY. 



Nothing can be more conclusive as to the value of the Arenig 

 group than the distinctive part played by its characteristic fauna 

 and its distinct stratigraphical position. Since the determina- 

 tion by Hicks of the St.-David's fauna, Mr. Marr of Cambridge 

 recognized some fossiliferous shales south-east of Caernarvon, in 

 three localities on the banks of the Sciont ; these shales contain 

 Arenig species. The Trilobita and Brachiopoda indicate Arenig 

 affinities ; and the Orthoceras caereesiense occurs only in the Arenigs 

 of Llanvirn, near St. David's. The genus Caryocaris (a new spe- 

 cies of which has occurred here, C. Marrii) hitherto has not occurred 

 out of the Skiddaw Slates of Cumberland (0. Wrightii being the 

 type). The associated Graptolites are those of the Skiddaw rocks 

 also, viz. Diclymograptus bijidus, D. indentus, and the Llandeilo form 

 D. Murchisonii. The genus Barrandia would help to place these 

 beds either in the Arenig or the lower part of the Llandeilo group. 

 This locality is suggestive of the Arenig or Llandeilo beds striking 

 from Caernarvon to Bangor and Aber, west of the great fault that 

 runs from Dingle on the S.W. to Aber on the N.E. 



Arenig of South Wales at St. David's. 



The following table enumerates all the species at present known 

 in the Arenig rocks of St. David's ; it is convenient to subdivide the 

 group into 3 series, both on lithological and pakeontological grounds. 

 It will be seen how distinct the fauna of each division or series ap- 

 pears. The total number of species at St. David's is 70. The lower 

 Arenig has yielded 16 species, the middle 24, and the upper 33. 

 14 species are peculiar or confined to the lower series, 21 to the 

 middle, and 30 to the upper. ISTo species out of the 70 occurs in all 

 three divisions. We thus see how distinctive a group of fossils 

 each subdivision contains. For the construction of the Arenig Table 

 (No. VIII., p. 115) I have selected 18 localities from which the Arenig 

 species have been obtained, and tabulated 8 of the most important. 

 The four type areas, where the Arenig rocks are present and well 

 developed, are : — 



1. Westmoreland: Skiddaw. 



2. Shropshire : Shelve area. 



3. North Wales : Tremadoc, Tai-hirion, &c. 



4. South Wales : Eamsey Island and St.-David's area. 



The chief localities for fossils are the following 18, comprised in 

 the above 4 areas : — 



1. Westmoreland: Skiddaw, Longside, Keswick, Outerside, Scaafel, White- 



side, and Braithwaite River. 



2. Shropshire : Shelve area (west of Stiperstones), Mytton Dingle, Perkins 



Beech. 



3. North Wales : Tremadoc, Garth, Ty-ohry, Tai-hirion. 



4. Sottth Wales : Eamsey Island, Llanvirn, Whitesand Bay, and St. David's. 



