These tiny organisms generally live in the open ocean at various depths, and sinking to the bottom, sometimes as deep as 2,000 to 4,000 fathoms, they form an ooze or mud. =Subdivisions.--= Radiolaria are divided into the four legions or orders,--Acantharia, Spumellaria, Nasselaria and Phaeodaria: only the second and third groups are found fossil. The Spumellarians are spherical, ellipsoidal, or disc-shaped, and the Nasselarians conical or helmet-shaped. =Cambrian Radiolaria.--= Certain cherts or hard, siliceous rocks of the palaeozoic era are often crowded with the remains of Radiolaria, giving the rock a spotted appearance. (See _antea_, Fig. 38). Some of the genera thus found are identical with those living at the present day, whilst others are peculiar to those old sediments. In Australia, remains of their siliceous shells have been found in cherts of Lower Cambrian age near Adelaide. These have been provisionally referred to the genera _Carposphaera_ and _Cenellipsis_ (Fig. 66). =Ordovician Radiolaria.--= Radiolaria have been detected in the Lower Ordovician rocks of Victoria, in beds associated with the Graptolite slates of this series. In New South Wales Radiolarian remains are found in the cherts and slates of Upper Ordovician age at Cooma and Mandurama. =Silurian Radiolaria.--= The Silurian black cherts of the Jenolan Caves in New South Wales contain casts of Radiolaria. =Devonian Radiolaria.--= The Lower Devonian red jaspers of Bingera and Barraba in New South Wales have afforded some casts of Radiolaria, resembling _Carposphaera_ and _Cenosphaera_. [Illustration: =Fig. 66.--FOSSIL RADIOLARIA.= A--Aff. Carposphaera (after David and Howchin). Cambrian. Brighton, S.A. B--Cenosphaera affinis, Hinde. Mid. Devonian. Tamworth, N.S.W. C--Amphibrachium truncatum, Hinde. Up. Cretaceous. Pt. Darwin. D--Dictyomitra triangularis, Hinde. Up. Cretaceous. Pt. Darwin. ] The large number of fifty-three species have been found in the radiolarian rocks of Middle Devonian age at Tamworth in New South Wales (Fig. 66). These have been referred to twenty-nine genera comprising amongst others, _Cenosphaera_, _Xiphosphaera_, _Staurolonche_, _Heliosphaera_, _Acanthosphaera_ and _Spongodiscus_. =Cretaceous Radiolaria.--= Although certain silicified rocks in the Jurassic in Europe have furnished a large series of Radiolaria, the Australian marine limestones of this age have not yielded any of their remains up to the present. They have been found, however, in the Lower Cretaceous of Queensland, and in the (?)Upper Cretaceous of Port Darwin, N. Australia. The Radiolaria from the latter locality belong to the sub-orders Prunoidea, Discoidea and Cyrtoidea (Fig. 66). The rock which contains these minute fossils is stated to be eaten by the natives for medicinal purposes. As its composition is almost pure silica, its efficacy in such cases must be more imaginary than real. =Cainozoic Radiolaria.--=