130 INVERTEBRATE PALAEONTOLOGY 



older strata. They immediately assumed biological 

 and geological importance. Alcyonaria (or Octocoralla) 

 have a like range, but are less satisfactorily known as 

 fossils. Lower Palaeozoic Zoantharia belonged chiefly 

 to two kinds, Tetracoralla (" Rugose Corals ") and 

 Tabulata. True Corals (in the recent sense) were want- 

 ing, unless the Archaeocyathidae belong to that group. 

 The precise affinities of both sub-classes are doubtful, 

 and recognition of the former is not universal. While 

 Tetracorals have much in common with the later Hexa- 

 coralla (and may perhaps be an early and peculiar mani- 

 festation of that group), Tabulates, with rudimentary or 

 absent septa, seem at first sight more akin to Alcyonaria 

 or Hydrozoa. Both groups were important rock-builders 

 in Lower Palaeozoic times, continuing through the Upper 

 Palaeozoic and then becoming virtually extinct. 



Relatively few Tetracorals are known from the 

 Ordovician ; small, solitary forms called Petraia are 

 the most abundant. Amplexus and Cyathophyllum also 

 occur, usually as small and simple species. In the 

 Silurian an extremely rich series appeared. Petraia is 

 still found in the Llandovery, while in the Wenlock Palaeo- 

 cyclidae, Cyathophyllidae and Cystiphyllidae formed 

 no inconsiderable bulk of the contemporary " coral-reefs." 

 Solitary types (e.g. Palaeocyclus, Cyathophyllum, Omphyma^ 

 Goniophyllum) were accompanied by massive colonies 

 (e.g. Cyathophyllum^ Strombodes (Pl.ix.fig. ^^Acervularia}. 



Tabulates were undoubtedly the predominant group 

 of Zoantharia in the Lower Palaeozoic. Favositids with 

 massive, fungiform colonies (e.g. Favosites, Alveolites) 

 are not uncommon in the Ordovician, and abound in the 

 Silurian ; Auloporidae, Syringoporidae and Halysitidae 

 with recumbent or fasciolate habit are scarce at the lower 

 horizon, but (especially the last-named) profuse in the 

 Silurian. The fibrous Chaetetidae are also known from 



