672 ON THE ARENIG AND LLANDEILO GRAPTOLITES OF ST. DAVID'S. 



Fig. 3. Dendrograptus serpens, Hopk., nat. size. From Abereiddy Bay : Lower 

 Llandeilo {coll. H. H.) Nearly the whole of the shale containing the 

 impression of this specimen is covered with the fine terminal branch- 

 lets ; but these are only represented in the figure where the actual 

 connexion between them and the main branches is clearly shown. 

 4. Bictyograptus, sp. ind., nat. size. From Abereiddy Bay : Lower Llan- 

 deilo {coll. J. K). 



Discussion. 



Mr. Carruthers said that this paper added greatly to our know- 

 ledge of the Graptolites. He had doubts as to the true position of 

 the Cladophora. Of the Rhabdophora the later forms seemed to be 

 simpler in their structure than the earlier ones. 



Mr. Hicks stated that the true Graptolites occur in the lowest 

 part of Ramsey Island, together with the dendroid forms. 



Prof. T. Rupert Jones inquired whether, if it were true that the 

 later forms of Graptolites were simpler than the older ones, we may 

 regard this as due to a degeneration leading towards an extinction 

 of the type. 



Mr. Hopkinson, in reply, stated that the dendroid forms are only 

 known to occur in abundance, in Britain, in the Arenig rocks of 

 St. David's, and that there are here intermediate forms connecting 

 British and American species, which occur in rocks of more ancient 

 date. He remarked that he did not consider the dendroid forms 

 valuable for determining zones, species very nearly allied to those of 

 the Arenig rocks being met with even in the Lower Ludlow rocks 

 of Shropshire ; but the Rhabdophora occur only in small zones, and 

 wherever they are found they seem to hold an equivalent position. 

 They are consequently valuable for stratigraphical purposes. Mr. 

 Hopkinson stated that in recent deep-sea dredgings Hydroids had 

 been found approaching the Graptolites in structure, and that Grap- 

 tolites have also lately been discovered which have many points in 

 common with the recent Sertularian Zoophytes. 



