Hydractinia dates from the Upper Cretaceous rocks in England, and in Australia its encrusting polypidom is found attached to shells in the polyzoal limestone of Mount Gambier (Miocene).
Stromatoporoids.
An important group of reef-builders in Palaeozoic times was the organism known as Stromatopora, and its allies. The structures of these hydroid polyps resemble successional and repetitional stages of a form like Hydractinia. As in that genus it always commenced to grow upon a base of attachment such as a shell, increasing by successive layers, until the organic colony often reached an enormous size, and formed great mounds and reefs (see antea, Fig. 32). The stromatoporoid structure was formed by a layer of polyp cells separated by vertical partitions, upon which layer after layer was added until a great vertical thickness was attained. This limestone-making group first appeared in the Silurian, and probably reached its maximum development in Middle Devonian times, when it almost disappeared, except to be represented in Carbopermian strata by a few diminutive forms.
Fig. 72—STROMATOPOROIDEA and CLADOPHORA.
A—Actinostroma clathratum, Nich. Devonian. Rough Range, W.A.
B—Actinostroma clathratum, Nich. Devonian. Rough Range, W.A. Vertical section.
(After G. J. Hinde).
(After T. S. Hall).
(After T. S. Hall).