48 OUR COMMON BRITISH FOSSILS. 



being placed back to back. (9) Cliniacogmptns ; a 

 genus so named by Professor Hall, of America, in 

 which the polypary has a double series of cells 

 hollowed out of the outer covering. (10) Lastly, we 

 have DicranograptiLS ; having double rows of cells in 

 the lower part, with branches possessed of only single 

 rows of cups or cells. 



In the Arenig rocks of Ramsey Island, the 

 dendroid forms seem to be tolerably abundant, and 

 Mr. Hopkinson has shown that these have a nearer 

 relation to the species of Graptolites in the Quebec 

 group of Canada than any other found in Great 

 Britain. The bilaterally branched or double form 

 of Graptolite seem to be peculiar to the Lower 

 Silurian rocks ; whilst the fewer species met with in 

 the upper strata are usually of a simpler character. 

 Some compound forms seem to have attained great 

 length ; thus, a species of Pleiirograptics has been 

 traced over three feet long, although even this does 

 not seem to have been the full size. The Skiddaw 

 Slates were formerly believed to form the lowest 

 horizon where the Graptolites were met with, but 

 Mr. Hopkinson's discovery of them, lower down in the 

 Arenig rocks, not only extends their antiquity, but, 

 owing to the similarity of type between the Arenig 

 species and those from Quebec, suggests that theii^ 

 geographical distribution into colonies occurred later 

 on through the subsequent geological changes which 

 took place. Still older species of Graptolites hav© 



