LOWER PALAEOZOIC FAUNAS 131 



these periods, but Chaetetes was more fully developed in 

 the Upper Palaeozoic. Favosites (PI. ix. fig. 4) and Haly- 

 sites (PL ix. fig. 5) were the chief reef-builders of the era. 



Alcyonaria were apparently represented by the 

 abundant and short-lived family Heliolitidae. Heliolites 

 persisted into the Devonian ; the majority ot genera 

 were restricted to the Lower Palaeozoic, attaining chief 

 prominence in the Silurian. 



There are two series of Palaeozoic organisms referred 

 to the Hydrozoa. Of these, the Stromatoporoidea have 

 had a chequered systematic history, having been classed 

 at various times as Calcareous Algae, Foraminifera, 

 Porifera, Zoantharia and Polyzoa. Massive, " spongy," 

 encrusting sheets of Clathrodictyon and Stromatopora 

 play an important part in the construction of Silurian 

 reefs, but the group was less abundant in the Lower 

 Palaeozoic than in the succeeding era. 



The Hydrozoan affinities of the Graptolitoidea are 

 tolerably established, but they have been regarded 

 as Plants, Porifera, Polyzoa and even Cephalopoda ! 

 Possessing wholly chitinous envelopes, Graptolites are 

 rarely well-preserved, but this failing in no way detracts 

 from their abundance and stratigraphical value. The 

 two orders, Dendroida and Graptoloida, seem to illustrate 

 evolutional principles in a convincing manner. The 

 former, with bush-like colonies supporting very small, 

 simple thecae, arose in the Upper Cambrian (Dictyonema] 

 and persisted to the Carboniferous, rarely attaining 

 abundance and showing little modification. The latter, 

 with less complex habit but far more elaborate thecae, 

 appeared in the Ordovician, and, with the exception of 

 one very rare type, became extinct in the Silurian. 

 Owing to their short and varied career, and prominent 

 acme, Graptoloida are of supreme importance as zonal 

 indices in the two periods of their existence. Most 



