THE VEGETATION OF FORMER TIMES. 611 



we might conclude that it originated from plants adhering to the limestone reefs 

 formed by animals or from sea-plants which lived on the borders of these limestone 

 cliffs. Where graphite is found in greatest quantity one is tempted to think it 

 might have been derived from peat moors. As we have said, all these are merely 

 suppositions, for since the carbon, lime, and silicates have become crystalline all 

 the points for the determination of the families to which the graphite- forming 

 plants belonged are lacking. It might be noted here, by the way, that although 

 graphite does indeed furnish the oldest traces of plant-life on the earth this does 

 not prove that the plants which gave rise to it were necessarily the first which 



Fig. 366.— Riella helicophylla growing under water. Enlarged. 



existed there. It is doubtful whether the rock which is associated with graphite 

 formed the first hard crust of the earth. Much more probably this rock was com- 

 posed of other broken rocks just as it has itself been again demolished, fux'nishing 

 the material for new strata. 



The shapes of vegetable remains from palaeozoic formations are fairly easily 

 recognizable. Those which were formerly regarded as fossil Sea-v/racks have 

 indeed been more recently interpreted as the trails of worms and medusae, but 

 some of them are without doubt the remains of Sea-wracks. The only other 

 known lowly plant which at that time had an aquatic habit is the curious 

 Spirophyton, the so-called Cock's-tail Alga (see accompanying fig. 365). This, 

 though some regard it as of purely inorganic origin, may pei'haps be regarded 

 as a submerged Liverwort; at any rate it is not without resemblance to Riella 

 Reuteri, which at the present time lives in the Lake of Geneva, and to the 

 Algerian Riella helicophylla (see fig. 366). No trace is to be found of Thallo- 



