n6 



PKOCEEDINGS OP THE GEOLOGICAL SOCIETY. 



Llandeilo. 



It may be thought by some superfluous to discuss the history of 

 the Llandeilo rocks proper ; but it is part of my purpose to do so, 

 for two reasons : — first, to show their true relations (affinities and 

 differences) to the Arenigs, which they overlie and with which they 

 have long been confounded ; and, secondly, to show the geographical 

 distribution of . the Llandeilo species and their palaeontological rela- 

 tions to the Caradoc rocks above and the Arenigs below. The three 

 classical localities for the Llandeilo rocks and fossils are Llandeilo, 

 Builth, and Abereiddy Bay north of St. David's. Each of these areas 

 possesses a characteristic and definite fauna ; on the whole the species 

 in each locality are much the same ; still the facies is peculiar in 

 each. It is also of much interest to clearly understand the true 

 position of the great band of dark roofing-slate of the " Ffestiniog 

 quarries " &c. that underlies the Caradoc or Bala series, occupying 

 the position of the Llandeilo flags. " It is probable that the lowest 

 portion of the Llandeilo group may occur in the Arenig and Ffesti- 

 niog mountains, and also in the Arrans ;" but the evidence is slight 

 and the materials (in the shape of fossils) are scanty. Salter obtained 

 Bellerophon perturbatus in dark slates near Bangor, and also other 

 fossils in the slates, identical with those of the Llandeilo rocks, 

 " which overlie the Arenig porphyries." It is doubtful if the 

 black slates of Anglesey are the equivalents of the dark earthy 

 slates that range east of the Arenig mountain. The Anglesey slates 

 contain Asaphus Powisii and Phacops apiculatus, both rare in the 

 Llandeilo group ; but the Asaphus occurs both in North and South 

 Wales, Phacops apiculatus only in Anglesey. The Graptolites , through 

 Climacograptus ccelatusfi) and Didymograptus Murchisoni, distinctly 

 enable us to refer the beds to the Llandeilo group. 



The Llandeilo rocks and their fossils in North Wales are confined to 

 few localities, and are but feebly exposed anywhere ; and the whole 

 fauna of this formation in that area numbers 47 species. Eighteen 

 of these belong to the prolific class Brachiopoda, 15 to the Crustacea. 

 The Llandeilo rocks of South Wales contain 88 species ; and about 

 40 are common to North and South Wales. It is important to show 

 those species which unite the two areas ; but for my purpose I name 

 only the Crustacea (Trilobita), Hydrozoa, and Brachiopoda. Of the 

 former class 8 species are common, viz. Asaphus tyrannus, A. 

 Powisii, Ogygia PortlocJcii, Calymene cambrensis, C. Blumenbachii, 

 Trinucleus concentricus, T. favus, and the Ostracode Beyrichia 

 complicata. 



The Hydrozoa are not less important ; they are Diplograptus foli- 

 aceus, D.dentatus, Didymograptus Murchisoni, Leptograptus flaccidus, 

 Dicranograptus ramosus, D. formosus, D. Nicholsoni, Climacograptus 

 bicornis, C. ccelatus, C. confertus, C. Sharenbergi, and Dicellograptus 

 sextans. 



The Brachiopoda number still more, as we should expect from 

 their habit and relation to the sea-beds upon which they live. They 

 are all common and well-known species ; and 8 of the 13 forms 



