AKNTVERSARY ADDRESS OE THE PRESIDENT. 



of these species are Hydrozoa, 2 are Crustacea (Trinucleus Ram- 

 sayi and Homalonotus bisulcatus), and 3 Brachiopoda (Lingula 

 brevis, L. attenuata, and OrtJiis calligramma). Looking, therefore, 

 at the number of genera (63) and species (150) in the whole Arenig, 

 the change or extinction at the close of the period has scarcely 

 any parallel in the Palaeozoic rocks. The accompanying Table 

 shows the numerical value of the Arenig species, the 63 genera and 

 150 species being distributed through four well-determined areas, 

 and showing the fauna to be, in the 



Skiddaw beds 31 genera and 49 species. 



Stijjerstones area . . 26 



North Wales 21 



South Wales 37 



41 

 26 

 76 



Only 8 genera and 9 species transgressed or passed to the suc- 

 ceeding formations ; and 11 genera and 16 species came in with 

 the advent of the group from the Lower Cambrian series. The 

 true numerical faunal value of the Arenig group is therefore 51 

 genera and 133 species ; those forms, or the 9 species, which pass to 

 higher formations do not affect the Arenig beyond linking it to the 

 next group in succession. 



Table VIII. — Arenig. 



£ 

 5 1 



Geographical 

 Distribution. 



Pass to 



Plantas 



Protozoa 



Hydrozoa 



Actinozoa 



Echinoderniata 



Annelida 



Crustacea 



Bryozoa 



Brachiopoda 



Lamellibranchiata 



Gasteropoda 



Pteropoda 



Heteropocla 



Cephalopoda 



18 



42 



150 



