The Hydrozoa are represented by the Grapto-lites, a series of forms which are confined to the older Palaeozoic rocks. Dictyonema (I, 6) is a complex Graptolite, found abundantly in a thin band of shale near the top of the Upper Cambrian, which is of nearly world-wide distribution. It shows the great value of the organisms which live at the surface of the open sea (pelagic fauna) in fixing contemporaneous deposits over enormous areas.

Other Hydrozoa are the jellyfish, of which recognizable casts have been found in large numbers. Stromatopora formed reefs in some of the Cambrian limestones.

It is still a question whether Corals were present in the Cambrian; certain fossils (Archceocyathus, I, 7, 8,) which by some authorities are called corals, are by others regarded as sponges. Though sufficiently abundant in some parts of the West to form reefs, the genus has only a few species, and, except locally, they are not conspicuous elements in the fauna.

The Graptolites, which were so abundant in the Ordovician and had become much less common in the Silurian, are now almost extinct, only a few simple species occurring in the Lower Devonian. The Corals, on the contrary, expand and multiply enormously both in numbers and in size. Most of the Silurian genera persist (though the chain-coral Halysites has become extinct), and many new forms are added. Heliophyllum (Fig. 266) is an example of the solitary corals, and Phillips-astrcea and Acervularia (Fig. 267) of the reef-builders.