PALAEOZOIC AGE 



ORDOVICIAN PERIOD 



LIFE in Ordovician times exhibited many developments, but 

 none of a sensational character. Remains have been found 

 of seaweeds in part lime-hardened, as is the case with Coral- 

 line seaweeds of our own tune (Solenopora, Rhabdoporella), 

 and giant forms of Laminaria were also in existence. 



Sponges were extending their range (Protospongia 

 fenestrata, etc.), and were larger, as a rule, and more 

 graceful than their Cambrian ancestors. Corals, simple 

 (Streptelasma) and compound (Columnaria, Cyathophyllum), 

 were numerous, and apparently possessed but trifling differ- 

 ences from some corals now living. The creatures, as a rule, 

 appear to have been in thick-walled, non-porous cups ; and 

 in the compound forms there was, therefore, an absence of 

 that free circulation which characterised many colonies of 

 later times. No reefs, it would seem, had yet been formed. 

 A few colonies apparently Alcyonarians were now to be 

 seen (Heliolites, Syringopora). These probably were fore- 

 runners of blue corals (Helipora), " sea-fans " (Gorgoniida), 

 and Organ-pipe coral (Tubipora). 



Colonies related to corals and of " sea-fir " type were 

 here much as those of the Cambrian. New colonies, how- 

 ever, known as Graptolites, now come into view. These, 

 no doubt, were related to the older colonies (Cladophora), 

 but had become distinguishable by several modifications. 

 The branching framework of the colony was somewhat 

 stronger, and able to bear a double-rowed population. Vari- 

 ous shapes were also developed so as to afford the tiny indi- 

 viduals greater facilities for obtaining food. Fixed existence 

 characteristic of the earlier colonies had also ceased. 



21 



SPONGES 



CORALS 



GRAPTO- 

 LITES 



