The Brittle-stars (Ophiuroidea) have their body covered with hard, calcareous plates. Their remains are found in rocks as old as the Ordovician in Bohemia but their history in Australia begins with the Silurian period (Fig. 29). From thence onward they are occasionally found in successive strata in various parts of the world. The bag-like echinoderms (Cystidea) form a rare group, restricted to Palaeozoic strata. The plates of the sack, or theca, and those of the slender arms are calcareous, and are capable of being preserved in the fossil state. A few doubtful remains of this group occur in Australia. The bud-shaped echinoderms (Blastoidea) also occur chiefly in Devonian and Carboniferous strata. This is also a rare group, and is represented by several forms found only in New South Wales and Queensland. The well known and beautiful fossil forms, the Stone-lilies (Crinoidea) have a very extended geological history, beginning in the Cambrian; whilst a few species are living in the ocean at the present day. The many-jointed skeleton lends itself well to fossilisation, and remains of the crinoids are common in Australia mainly in Palaeozoic strata (Fig. 30). In Europe they are found abundantly also in Jurassic strata, especially in the Lias. [Illustration: =Fig. 30.= =A Fossil Crinoid (Taxocrinus simplex, Phillips sp.)= About 1/2 nat. size. Wenlock Limestone (Silurian), Dudley, England. (_Nat. Mus. Coll._) ] [Illustration: =Fig. 31.--Graptolites on Slate (Tetragraptus fruticosus, J. Hall, sp.)= Nat. Size. Lower Ordovician. Bendigo, Victoria. (_Nat. Mus. Coll._) ] [Illustration: =Fig. 32.= =Polished Vertical Section of a Stromatoporoid. (Actinostroma).= Nat. size. Middle Devonian. South Devon, England. (_F.C. Coll._) ] HYDROZOA.--The Graptolites ("stone-writing") have a chitinous skin (periderm) to the body or hydrosome, which is capable of preservation to a remarkable degree; for their most delicate structures are preserved on the surfaces of the fine black mud deposits which subsequently became hardened into slates. In Australia graptolites occur from the base of the Ordovician to the top of the Silurian (Fig. 31). 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). [Illustration: =Fig. 33.--Fossil Corals (Favosites).= Photograph of a Polished Slab, 2/3 nat. size. In Devonian Limestone, Buchan, Victoria.] [Illustration: =Fig. 34.--Siliceous Skeleton of a Living Hexactinellid Sponge.= Probably Chonelasma.