IOO 



PKOCEEDINGS OF THE GEOLOGICAL SOCIETY. 



Table or record of the earliest rocks known, the numerous blanks 

 show that the classes were not represented or had not appeared 

 in the British Isles ; but as we proceed through the formations we 

 find that in time, or as life progresses, all the classes become repre- 

 sented. The study of these Tables with the description may be 

 useful ; and it is hoped they will be found not far from the truth, 

 and exhibit the present state of our knowledge of the British species 

 and their distribution in time. 



Table VII. 



Classes. 



Plants? 



Protozoa 



Actinozoa. 

 Eckinoderniata . . . 



Annelida 



Crustacea 



Bryozoa , 



Bracliiopoda 



Lamellibranchi ata 

 Gasteropoda. 



Pteropoda 



Heteropoda 



Cephalopoda 



o 



O 



3 



9 



12 



28 



103 



1 



1 



5 



20 



5 



12 



4 



17 



1 



6 



9 



2 







! -QD 

 S3 



5 8 2P ® SP 

 s <3 P 



o 



CD oS 



* s 



61 182 



EH 8 

 P 



Arenig Rocks. 



The Arenig or Skiddaw group of Sedgwick, immediately under- 

 lying the Llandeilo flags, forms the base of the true Silurian rocks. 

 From Arenig-fawr Prof. Sedgwick and Mr. Salter obtained two of 

 the characteristic fossils, Calymene parvifrons and Ogygia Selwyni. 

 Sedgwick recognized these beds as being different from the Llandeilo 

 above, and called them Arenig slates, believing them at the time to 

 be the top of his Ffestiniog group. The group is distinctly recog- 

 nized in the lead-mining district of the Stiperstones area, east of 

 the Longmynd, underlying the true Llandeilo flags of Shelve and 

 Corndon Hill, the Lower Llandeilo of Murchison. Salter first recog- 

 nized the Arenig group near St. David's, passing upwards into the 

 Llandeilo flags, which are so finely shown at Abereiddy Bay. The 



