Clay ironstone nodules (sphaerosiderite) have generally been formed as accretions around some decaying organic body. Many clay ironstone nodules, when broken open, reveal a fossil within, such as a coprolitic body, fern frond, fir-cone, shell or fish. Oolitic ironstones are composed of minute granules which may have originally been calcareous grains, formed by a primitive plant or alga, but since replaced by iron oxide or carbonate. The Tertiary ironstone of western Victoria is found to contain leaves, which were washed into lakes and swamps (Fig. 53); and the ferruginous groundmass may have been originally due to the presence of diatoms, though this yet remains to be proved. PART II.--SYSTEMATIC PALAEONTOLOGY. CHAPTER V. FOSSIL PLANTS. =Cambrian Plants.--= The oldest Australian plant-remains belong to the genus _Girvanella_. This curious little tubular unicellular organism, once thought to be a foraminifer, shows most affinity with the blue-green algae (Cyanophyceae), an important type of plant even now forming calcareous deposits such as the calcareous grains on the shores of the Salt Lake, Utah, and the pea-grit of the Carlsbad hot springs. _Girvanella problematica_ occurs in the Lower Cambrian limestones of South Australia, at Ardrossan and elsewhere. =Silurian Plants.--= Amongst Silurian plants may be mentioned the doubtful sea-weeds known as _Bythotrephis_. Their branch-like impressions are fairly common in the mudstones of Silurian age found in and around Melbourne. They generally occur in association with shallow-water marine shells and crustacea of that period. The genus _Girvanella_ before mentioned is also found in the Silurian (Yeringian) of Lilydale and the Tyers River limestone, Victoria (Fig. 54). [Illustration: =Fig. 54.--Section through pellet of Girvanella conferta=, Chapm. × 35. From the Silurian (Yeringian) Limestone of Tyers River, Gippsland, Victoria. (_Nat. Mus. Coll._) ] _Haliserites_ is a primitive plant of the type of the club-mosses so common in the rocks of the Carboniferous period. This genus is found in some abundance in the Yeringian stage of the Silurian in Gippsland (Fig. 55). [Illustration: =Fig. 55.--PALAEOZOIC PLANTS.= (Approximate dimensions in fractions). A--Bythotrephis tenuis, J. Hall. Silurian. Victoria. B--Haliserites Dechenianus, Göppert. Silurian. Victoria. C--Cordaites australis, McCoy. Upper Devonian. Victoria. D--Sphenopteris iguanensis, McCoy. Upper Devonian. Victoria. E--Glossopteris Browniana, Brongniart. Carbopermian. N.S.W. ] [Illustration: =Fig. 56.= =Restoration of Lepidodendron elegans.= (_After Grand'Eury._) ] [Illustration: =Fig. 57.= =Lepidodendron australe, McCoy.= Portion of Stem showing Leaf-cushions. Slightly reduced. Carboniferous. Manilla River, Co. Darling, N.S.W.