334 PALEONTOLOGY 



rock-builders in the past. Their remains have often 

 been misunderstood and attributed to Foraminifera, 

 Sponges or Hydrozoa. The calcareous green seaweeds 

 (Chlorophyceae) form in general branching tubular 

 structures, which often break up into segments of 

 characteristic appearance. Such are Gyropovella (Perm.- 

 Trias.) and Diplopora (Trias.), which play a part in the 

 building of the Alpine dolomites, and Ovulites (Eocene), 

 of which the segments, common in the calcaive grassier of 

 Paris, are egg-shaped with a central tubular cavity. 



The calcareous red seaweeds, on the other hand, are 

 more often massive or encrusting. Important are 

 Solenopora (Camb.-Jur., Fig. 100, a), which contributes to 

 the substance of many British Palaeozoic Limestones, and 

 Lithotliamnion (Cret.-Rec.), a rock-former in the Miocene 

 of the Vienna basin. 



An interesting group, which attains a higher grade 

 than the true Algae, though on different lines from 

 other plants, is the Characea, of freshwater habitat and 

 secreting a calcareous skeleton. It is best known by its 

 spirally-marked fruits, found in the Purbeck and Oligocene 

 beds of the South of England (Fig. 100, b). 



Pteridophyta (Vascular Cryptogams). Three distinct 

 lines of descent may be recognized, in all three of which 

 two sub-grades are passed through (a lower homosporous, 

 with only one kind of spore and sporangium, and a 

 higher heterosporous, with mega and micro-sporangia and 

 spores), while two of them evolve independently into 

 the higher seed-bearing grade. These three lines are 

 the Lycopodiales (represented to-day by the creeping, 



