io6 



PKOCEEDLNGS OF THE GEOLOGICAL SOCIETY. 



important as bearing both upon distribution and correlation. Again, 

 the Gasteropoda first appear in Britain in the Upper Arenigs of St. 

 David's (Pleurotomaria llanvirnensis, and Ophileta or RapJiistoma). 

 Yet, strangely, we have no record of any Lamellibranchs through 

 the whole of the Arenig group in South Wales ; 4 species are known 

 from the Stiperstones area, and 2 from Ty-obry, in North Wales. It 

 will be remembered that no less than 12 species occur below in the 

 Lower Tremadoc rocks of the same area. The value of this upper 

 division of the Arenig group in South Wales is enhanced by its re- 

 semblance to Barrande's Etage D 1 (Bohemia), and its close relation 

 to the fauna found in the Arenig slates in North-west Prance. The 

 recognized presence of this stage in Britain enables us to correlate 

 our beds with those on the continent, and thus establish a general 

 succession for the Western-European and Western-British rocks of 

 this age. 



This result is due to the researches of Dr. Hicks, as detailed in 

 his valuable papers upon the history and succession of the ancient 

 rocks of St. David's. Carefully constructed tables, prepared for the 

 purpose of testing the value of the Arenig group and its relation to 

 the Tremadoc below and Llandeilo above, clearly show that it stands 

 almost alone ; for of the 55 known species of Hydrozoa in the Arenig, 

 only 2 species pass to the Llandeilo {Didymograptus Murchisoni and 

 Climacograptus confertus). The first appearance and the abundance 

 of the Bhabdophora in the Arenig rocks would alone justify the posi- 

 tion now assigned to it as the base of the Lower Silurian rocks ; only 

 1 species is known to occur in the Tremadoc group (Dendrograptus, 

 from the Shineton Shales of Shropshire). The crustacean fauna is 

 equally conclusive ; out of 50 Arenig forms known, only 2 (Trinu- 

 cleus llamsayi, Hicks, and Homalonotus bisidcatus, Salt.) unite the 

 Arenig and Llandeilo through the Trilobita. Out of the 18 species 

 of Brachiopoda, only 3 pass to the Llandeilo, viz. Lingida brevis, 

 L. attenuata, and Orthis ccdligramma. No Lamellibranch, Pteropod, 

 Heteropod, Gasteropod. or Cephalopod (of which united, there are 

 30 species) passes to the Llandeilo. Thus, out of the known fauna 

 of 149 species, only 16 pass upwards into or are common to the 

 Llandeilo beds. 150 species make up the entire known fauna of 

 the Arenigs ; their distribution is as follows : — 125 species occur 

 in the North and South Wales Arenigs, 34 from various localities 

 in North Wales (Tremadoc, Ty-obry, and Tai-hirion), and 94 in 

 South Wales ; the Skiddaw species number 50 and the Stiper- 

 stones 36. These two latter localities are important ; they are widely 

 separated, and very few species occur in common. Of the 50 Skid- 

 daw species of all classes, only 9 occur in the area west of the 

 Stiperstones in Shropshire ; this is due chiefly to the rich Grap- 

 tolite fauna, which group is largely represented, no less than 52 

 species occurring in the Skiddaw beds alone. West of the Stiper- 

 stones only 6 species are known ; and they have an important bear- 

 ing upon distribution : they are Didymograptus patidus, Hall, D. 

 geminus, D. hirundo, D. constrictus, D. Murchisoni, and Clematograp- 

 tus implicatus. Diplograptus dentatus also occurs at Tremadoc and 



