It is probable that this order of Alcyonaria was better represented on the reefs of some of the earlier periods of the world's history than it is at present. The fossil _Syringopora_, which is found abundantly in the carboniferous limestone and other strata, was probably an Alcyonarian belonging to this order. It resembles _Tubipora_ in its mode of growth, but in place of the horizontal platforms connecting the zooids there are rods or bars from which new zooids spring (Fig. 152). Similar connecting bars are found in the recent _Clavularia_ (_Hicksonia_, Delage) {344}_viridis_ of the East Indian reefs (Fig. 153). Other fossil forms belonging to the order are _Favosites_, a very abundant coral of the Upper Silurian rocks, and possibly _Columnaria_. [Illustration: FIG. 152.—_Syringopora_, a fossil, showing autothecalous tubes (_th_), funnel-shaped tabulae (_tab_), and tubular cross-bars (_t_).] [Illustration: FIG. 153.—_Clavularia_ (_Hicksonia_) _viridis_, with creeping stolon and transverse connecting tubes.] The principal families of the Stolonifera are:— Fam. 1. CORNULARIIDAE.—Without spicules; _Cornularia_, Lamarck, Mediterranean; _Stereosoma_, Hickson, Celebes. Fam. 2. CLAVULARIIDAE.—_Clavularia_, Quoy and Gaimard; _Sarcodictyon_, Forbes, British; _Sympodium_, Ehrb.; _Syringopora_, Goldfuss, fossil. Fam. 3. TUBIPORIDAE.—_Tubipora_, Linnaeus, tropical shallow water. Fam. 4. FAVOSITIDAE.—_Favosites_, Lamarck; _Syringolites_, Hinde; _Stenopora_, King. ORDER II. COENOTHECALIA. This order contains the single genus and species _Heliopora coerulea_ among recent corals, but was probably represented by a large number of genera and species in earlier periods. {345}It is found at the present day in many localities in the warm shallow waters of the tropical Pacific and Indian Oceans. It usually flourishes on the inside of the reef, and may form masses of stone five or six feet in diameter. The coral may easily be recognised, as it is the only one that exhibits a blue colour. This colour usually penetrates the whole skeleton, but in some forms is absent from the superficial layers.