46 OUR COMMON BRITISH FOSSILS. 



fossilized. So far, their development by budding 

 after the manner of the Sargassum t or "gulf-weed," 

 would appear to be probable. Mr. John Hopkinson, 

 F.G.S., has shown that Graptolites may be grouped 

 into two great divisions, in one of which a fibrous rod 

 strengthened the outside of the single Graptolites, or 

 was in the centre in the double species. This rod, 

 Nicholson thinks, was originally hollow, and filled in 

 with living material. It must not be confounded, 

 however, with the hollow space (ccenosarc) communi- 

 cating with each cup, which was undoubtedly filled 

 with the " common flesh." The Graptolites which 

 possessed these rods are called Rhabdophores : all of 

 them were free and unrooted. But there is another 

 distinct group of Graptolites of simpler structure, 

 always branched, and " dendroid," or " tree-shaped," 

 like the Sertularians. These are termed Cladophora, 

 by Mr. Hopkinson, who has shown that they were 

 fixed or rooted, like the " sea-firs," and probably were 

 very similar to them. 



The reader who perceives the nature of the discus- 

 sion which has contributed so many opinions to the 

 natural-history relations of these interesting fossils, 

 will arrive at the conclusion that their nearest living 

 allies are the " corallines," or " sea-firs " (Sertularidce], 

 although they probably had strong affinities with a 

 group of animals even lower in organization than the 

 latter, namely, the Rhizopoda, of which Sponges and 

 Foraminifera are examples. 



