Fig. 31—Graptolites on Slate (Tetragraptus fruticosus, J. Hall, sp.)
Nat. Size. Lower Ordovician. Bendigo, Victoria.
(Nat. Mus. Coll.)
Fig. 32.
Polished Vertical Section of a Stromatoporoid. (Actinostroma).
Nat. size. Middle Devonian. South Devon, England.
(Nat. Mus. 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]).