The Middle Devonian strata of Bindi, Victoria, yield large, massive examples of _Actinostroma_. This genus is distinguished from the closely allied _Clathrodictyon_ by its vertical pillars passing through several laminae in succession. Rocks of the same age in Queensland contain _Stromatopora_, whilst in Western Australia the Rough Range Limestone has been shown to contain _Actinostroma clathratum_ (Fig. 72 A, B) and _Stromatoporella eifeliensis_. Cladophora. =Palaeozoic Cladophora.--= Some branching and dendroid forms of Hydrozoa probably related to the modern Calyptoblastea ("covered buds"), such as _Sertularia_ and _Campanularia_, are included in the Cladophora ("Branch bearers"). They existed from Cambrian to Devonian times, and consist of slender, forking branches sometimes connected by transverse processes or dissepiments, the branches bearing on one or both sides little cups or hydrothecae which evidently contained the polyps, and others of modified form, perhaps for the purpose of reproduction. The outer layer, called the periderm was of chitinous material. They were probably attached to the sea-floor like the Sertularians (Sea-firs). =Dictyonema and Allies.--= Remains of the above group are represented in the Australian rocks by several species of _Dictyonema_ (Fig. 72 E, F) occurring in the Lower Ordovician of Lancefield, and in similar or older shales near Mansfield. Some of these species are of large size, _D. grande_ measuring nearly a foot in width. The genera _Callograptus_, _Ptilograptus_ (Fig. 72 C, D) and _Dendrograptus_ are also sparsely represented in the Upper Ordovician of Victoria, the two former from San Remo, the latter from Bulla. Graptolites (Graptolitoidea).-- =Value of Graptolites to Stratigraphist.--= The Graptolites were so named by Linnaeus from their resemblances to writing on the slates in which their compressed remains are found. They form a very important group of Palaeozoic fossils in all parts of the world where these rocks occur, and are well represented in Australasia. The species of the various Graptolite genera are often restricted to particular beds, and hence they are of great value as indicators of certain horizons or layers in the black, grey or variously coloured slates and shales of Lower Ordovician to Silurian times. By their aid a stratum or set of strata can be traced across country for long distances, and the typical species can be correlated even with those in the older slates and shales of Great Britain and North America. =Nature of Graptolites.--=