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PBOCEEDINGS OE THE GEOLOGICAL SOCIETY. 



to this division. The Trilobites in the Dolgelly area are Conocoryphe 

 abdita, Salt., C. Williamsonii, Belt, 0. longispina, Belt, Olenus 

 scarabceoides, Sphcerophthalmus {Olenus) cdatus, S. (Olenus) humilis, 

 Phill., Agnostus princeps, Salt., A. trisectus, Salt., and A. obtusus, 

 Belt ; and, as before stated, the Malvern species are equally restricted. 

 No one of these occurs below in the Middle and Lower Lingula-flags. 

 Orthis lenticularis, Dalm., and Obolella follow the same rule all 

 through the Maentwrog, Ffestiniog, and Dolgelly deposits ; the Tri- 

 lobita that successively appear are closely allied, or belong to the 

 Agnostidse and Olenidse ; no new types of structure come in, the 

 fauna being distinctive and similar and homogeneous as a whole. 

 Dikelocephalus, Conocoryphe, Olenus, and Paradoxides constitute the 

 natural group or family Olenidae ; and the single genus Agnostus is 

 rich in species. The succeeding Tremadoc and Arenig rocks evidence 

 great and sudden change, faunal and otherwise, the large Asaphidae, 

 Cheiruridae, Trinucleidae, and Calymenidee then first appearing; 

 sixteen genera at once occur in the Arenig, and two in the Tremadoc 

 (Niobe and Psilocephalus) ; and we have no proof of any unconformity 

 in either the North or the South Wales areas : these large forms 

 come in at once and in vast numbers ; and the old forms die out.' 

 Nor have we as yet any evidence of these new and distinct types 

 having had prior existence in some other area, and migrated into 

 the Tremadoc or Arenig seas ; we know not at present where to look 

 for evidence of this. Ireland possesses no rocks or fossils of this age 

 or earlier, Scandinavia none, Western Europe none. America then 

 possessed gigantic Asaphi ; but in which direction dispersion or dis- 

 tribution may have taken place we have no trace ; although the 

 evidence tends to show that it must have been {towards the east, 

 or Europe, the main strike of the oldest American rocks being N.E. 

 and S.W. ; while the older groups are greatly developed on the 

 eastern side of the North- American continent, and there is con- 

 siderable affinity between the faunas of the two areas, the facies 

 strongly resembling each other. No migration of an earlier fauna 

 from one area would fully account for the disappearance of the 

 earlier gigantic Trilobites (such as Paradoxides, Plutonia) and other 

 equally characteristic genera (such as Erinnys, Holocephalina, &c), 

 all of which characterize the lowest rocks of Britain, ranging 

 from the Harlech to the top of the Lingula-flags, call them what 

 we will. 



Salter, as far back as 1853, most carefully described the two groups 

 (Lower and Upper) of the Lingula-flags in North Wales ; he ex- 

 amined them in their most typical localities, selecting Maentwrog, 

 Tremadoc, Efestiniog, Dolgelly, Carnedd Efiliast, Bangor, &c. 

 as those places where the Lower division could be best studied. 

 This he divided into two sections, the lowest consisting of black 

 pyritous slates, with numerous beds of intercalated sandstones near 

 the base. These probably in part were Menevian ; but the prevail- 

 ing fossils are Agnostus princeps, Olenus cataractes, and Lingulella 

 Davisii. (Salter's species A. princeps is the A. pisiformis, Linn., 



