Geological Society of London. 133 



Graptolites and their allies, which were almost certainly fixed, and 

 are most nearly allied to the recent ThecapJiora. 



The distribution of the genera and species in the Arenig and 

 Llandeilo rocks of St. David's was then treated of, and the different 

 assemblages of species in each of their subdivisions were compared 

 with those of other areas. 



The Arenig rocks are seen to contain a number of species which 

 ally them more closely to the Quebec group of Canada than to any 

 other series of rocks, all their subdivisions containing Quebec 

 species, while the Skiddaw Slates, which before the discovery of 

 Graptolites in the Lower Arenig rocks of Eamsey Island in 1872 

 were considered to be our oldest Graptolite-bearing rocks, can only 

 be correlated with the Middle and Upper Arenigs of St. David's. 

 The Graptolites of the Arenig rocks of Shropshire and of more dis- 

 tant localities were also compared with those of St. David's. 



In the Llandeilo series of this district the Cladophora have now 

 for the first time been found, a few species, with several species of 

 Bhabdophora, occurring at Abereiddy Bay in the Lower Llandeilo, 

 which alone has been carefully worked, there being much more to 

 be done in the Middle and Upper Llandeilo, from which very few 

 species of Graptolites have as yet been obtained. 



Some of the recently introduced terms, and altered or more definite 

 terminology, employed in the descriptions of the species were then 

 explained ; and the paper concluded with descriptions of all the 

 species of Graptolites collected in the Arenig and Llandeilo rocks of 

 St. David's within the last few years of which sufficiently perfect 

 specimens have been obtained, doubtful species being referred to in 

 an appendix. 



Forty-two species were described belonging to the following 

 genera: — Bidymogr aphis, Tetragraptus, Clemagraptus (gen. nov.), 

 Dicellograptus, Climacograptus, Diplograptus, Phyllogr aphis, Glosso- 

 graptus, and Trigonograptus (Bhabdophora) ; Ptilographis, Dendro- 

 graphis, Callograptus, and Dictyogr aphis (Cladophora). 



Discussion. — Mr. Carruthers said that this paper greatly added 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 branching forms 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 de- 

 generation 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 the British and American species which 

 occur in rocks of more ancient age. 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 Shrop- 

 shire ; 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 Graptolites have also lately been discovered which have many points in 

 ■common with the recent sertularian Zoophytes. 



