ANNIVERSARY ADDRESS OF THE PRESIDENT. 



95 



as the first zoological or palseontological break between the Lower and 

 Upper Cambrian, or the Cambrian and Silurian, takes place here, 

 and that without any stratigraphical or physical unconformity. INo 

 greater faunal change is known to take place through the whole of 

 the Pakeozoic rocks than at the close of the Tremadoc or Lower 

 Cambrian. 



Bryozoa. — None known. 



Brachiopoda. — Only 4 genera and 7 species occur ; and 6 of these 

 pass to the Arenig. They have therefore no stratigraphical value. 

 Obolella Belti is the single remaining or restricted form. 



Lamellibranchiata. — None yet known. 



Gasteropoda. — None have yet occurred below the Arenig. 



Pteropoda. — Conularia and Theca — the first with 1 species, O. 

 Homfrayi, which is also Arenig : the latter contains 6, one of which 

 {Theca simplex) also passes to the Arenig. 



Heteropoda. — Bellcrophon arfonensis, Salt., and B. midtistriatus, 

 Salt., are the only Upper-Tremadoc species. The last-named spe- 

 cies is Arenig also. 



Cephalopoda. — Cyrtoceras prcecox, Salt., and Orthoceras sericeum, 

 Salt.; the last also passes to the Arenig ; they are the first Cephalo- 

 pods known ; and it is doubtful if Cyrtoceras prcecox is not Lower 

 Tremadoc also. The rich locality Garth yields 0. sericeum from the 

 upper part of the Upper Tremadoc. The Arenig of Llanvirn at 

 St. David's has yielded the same ; it is therefore both a ZSTorth- 

 and South-Wales species, from the Tremadoc in one area, the Arenig 

 in the other. 



Table YI. — Upper Tremadoc. 



From 

 Lower 

 Tremadoc. 



Classes. 



Genera. 



Species. 



Pass to 

 Arenig. 





Plantse. 











Protozoa. 











Hydrozoa. 











Actinozoa. 











Echinodermata. 











Annelida. 









4 

 4 





11 



15 



4 



6 





Bryozoa. 









4 

 C 





4 



7 



3 

 6 





Lamellibranchiata. 











Gasteropoda. 









1 

 3 





2 



7 



3 



2 







1 



2 



1 







2 



2 





D 



1 3 





20 



33 



1 1 

 I 6 



