Another section of the Hydrozoa is the Stromatoporoidea. These are essentially calcareous, and their structure reminds one of a dense coral. The polyps build their tiers of cells (coenosteum) in a regular manner, and seem to have played the same part in the building of ancient reefs in Silurian, Devonian and Carboniferous times as the Millepora at the present day ([Fig. 32]).


Fig. 33—Fossil Corals (Favosites).
Photograph of a Polished Slab, 2/3 nat.
size. In Devonian Limestone,
Buchan, Victoria.


Fig. 34.

Fig. 34—Siliceous Skeleton of a Living
Hexactinellid Sponge.
Probably Chonelasma.
× 4. Mauritius. (Viewed in Two
Directions.)

(F.C. Coll.)

ANTHOZOA.—The true Corals have a stony skeleton, and this is capable of easy preservation as a fossil. There is hardly any fossiliferous stratum of importance which has not its representative corals. In Australia their remains are especially abundant in the Silurian, Devonian ([Fig. 33]), and Carboniferous formations, and again in the Oligocene and Miocene.

SPONGES.—The framework of the sponge may consist either of flinty, calcareous, or horny material ([Fig. 34]). The two former kinds are well represented in our Australian rocks, the first appearing in the Lower Ordovician associated with graptolites, and again in the Cretaceous and Tertiary rocks ([Fig. 35]); whilst the calcareous sponges are found in Silurian strata, near Yass, and again in the Cainozoic beds of Flinders, Curlewis and Mornington in Victoria.


Fig. 35.
Spicules of a Siliceous Sponge
(Ecionema newberyi, McCoy sp.)
Highly magnified. Cainozoic
Shell-Marl.
Altona Bay Coal-Shaft.
Fig. 36.
Nummulites (N. gizehensis Ehr. var.
champollioni, de la Harpe).
About nat. size. Middle Eocene Limestone.
Cyrene, Northern Africa.
(Coll. by Dr. J. W. Gregory).

PROTOZOA.—The important and widely-distributed group of the Foraminifera (“hole-bearers”) belonging to the lowest phylum, the Protozoa, generally possess a calcareous shell. The tests range in size from tiny specks of the fiftieth of an inch in diameter, to the giant Nummulite, equalling a five shilling piece in size ([Fig. 36]). Their varied and beautiful forms are very attractive, but their great interest lies in their multifarious distribution in all kinds of sediments: they are also of importance because certain of the more complex forms indicate distinct life zones, being restricted to particular strata occurring in widely-separated areas.

Fig. 37—Siliceous Skeletons of Radiolaria.
× 58. Late Cainozoic Age. Bissex Hill, Barbados, West Indies.

(F.C. Coll.)