104 PROCEEDINGS OE THE GEOLOGICAL SOCIETY. 



ranges under the foot of Moelwyn to the Manod mountains on the 

 east. This zone, all along its course, indicates and ushers in a new 

 set of conditions, physical and palseontological. No Graptolites 

 occur below this line anywhere ; but in the black slates immediately 

 overlying it at Garth &c. they occur in plenty, associated with cer- 

 tain species of Homalonotus, Asaphus, Calymene, JEglina, and Bio- 

 nide, accompanied by Conularia and Lamellibranchs ; and among the 

 lower beds of the volcanic rocks of the-Manods Calymene par vifrons 

 and Trinucleus Murchisoni occur, accompanied still further eastward 

 by Ogygia Selwynii, the species, of all others, which characterizes the 

 Arenig rocks of Shelve and Skiddaw, and occurs at Llanfaelrhys, in 

 South Caernarvonshire, associated with Lingula attenuata and Gfrap- 

 tolithus Murchisonii. This classification is borne out by the persis- 

 tent position of the grit which everywhere separates the two hori- 

 zons palseontologically as well as stratigraphically, the mass of the 

 Arenig with its peculiar fauna not occurring until the grit is 

 passed. No species of Ehabdophora is known below this grit (their 

 first appearance being in the Arenig rocks); and although the 

 Arenig Graptolithidse are rare in North Wales (4 species), as com- 

 pared with Skiddaw (27 species), or with South Wales, St. David's 

 (18 species), still the distinct Crustacea must be held as significant. 

 Again, comparing the value of the same formation in North and 

 South Wales, it is important to remember that, of the whole Arenig 

 fauna in South Wales (97 species), only 5 north-west forms are 

 common to the two areas : these are Calymene parvifrons, Ogygia 

 Selwynii, JEglina caliginosa, Lingulella Davisii, and Orthis lenticu- 

 laris. 26 Trilobites, 8 Brachiopoda, 38 Ehabdophora, and 3 Ptero- 

 poda are peculiar and confined to South Wales. Thus amongst the 

 same group of rocks the correlation of species is a difficult question, 

 even over so small a geographical area as Wales. 



The dark slates at Ty-obry, which immediately overlie the grit, 

 contain Calymene parvifrom, ^Eglina caliginosa, and Dionide atra, 

 associated with Diplograptus mucronatus, Climacograptus confertus, 

 and Glossograptus ciliatus, all Lower-Arenig species. Palaiarca 

 socialis and a species of Ctenodonta are the only North- Welsh 

 bivalves, and Conularia corium and C. margaritifera the only two 

 Arenig Pteropods known in North Wales. None of these occur in 

 the Arenig series of St. David's, and none pass to the Llandeilo 

 group in any area. 



The researches of Dr. Hicks at St. David's have resulted in greatly 

 advancing our knowledge of Sedgwick's group in that area*. In 

 his paper upon " the Succession of the Ancient Eocks of St. David's," 

 Dr. Hicks draws attention to the history of the names Arenig and 

 Llandeilo groups, and explains the manner in which the two series 

 were confounded by one of the authors who have written upon them, 

 and how the slates above and below the Arrans and Arenigs were 

 by him and the Geological Survey classed as equivalent to Llandeilo 

 flags of Builth &c. 



The St.-David's Arenigs consist mostly of black slates, resting 

 * Quart. Journ. Geol. Soc. vol. xxxi. pp. 167-194. 



