72 THE STORY OF THE EARTH AND MAN. 



also great multitudes of those little crustaceans which 

 are inclosed in two horny or shelly valves like a 

 bivalve shell-fish, and the remains of which sometimes 

 fill certain beds of Silurian shale and limestone. 



No remains found in the Silurian rocks have been 

 more fertile sources of discussion than the so-called 

 Graptolites, or written stones a name given long ago 

 by Linnaeus, in allusion to the resemblance of some 

 species having rows of cells on one side, to minute 

 lines of writing. These little bodies usually appear 

 as black coaly stains on the surface of the rock, 

 showing a slender stem or stalk, with a row of little 

 projecting cells at one side, or two rows, one on 

 each side. The more perfect specimens show that, in 

 many of the species at least, these fragments were 

 branches of a complex organism spreading from a 

 centre; and at this centre there is sometimes per- 

 ceived a sort of membrane connecting the bases of 

 the branches, and for which various uses have been 

 conjectured. The branches themselves vary much in 

 different species. They may be simple or divided, 

 narrow, or broad and leaf-like, with one row of cells, 

 or two rows of cells. Hence arise generic distinc- 

 tions into single and double graptolites, leaf and tree 

 graptolites, net graptolites, and so on. But while it 

 is easy to recognise these organisms, and to classify 

 them in species and genera, it is not so easy to say 

 what their affinities are with modern things. They 

 are exclusively Silurian, disappearing altogether at 

 the close of this period, and, so far as we know, not 



