A. EARLAND ON RADIOL ARIA. 281 



tjiich large numbers of Kadiolarian skeletons (as mucli as 75 per 

 cent, in some cases), that the deposit may be termed a pure 

 Eadiolarian ooze. At present no such deposit is known in 

 the Atlantic Ocean. In the natural state Eadiolarian ooze is 

 yellowish or brown in colour, from the admixture of minerals 

 which it contains, bat when decalcified it appears as a fine white 

 powder. 



Radiolaria, or, more correctly, Polycystina, have been found 

 in the fossil state in sedimentary deposits of all ages from the 

 ►Silurian and Cambrian to the present time. The great majority 

 of the fossil Eadiolaria, which all belong to the two legions 

 ^pumellaria and Nassellaria, are derived from Tertiary strata. 

 The richest and most important of these deposits are of Miocene 

 age, the best known being those of Barbados, Sicily, and the 

 Nicobar Islands, all of which may be regarded as pure Eadiolarian 

 oozes in a fossil condition. Besides these pure Eadiolarian oozes, 

 there are many other deposits known especially in the south of 

 Europe and north of Africa, in which Eadiolaria exist in con- 

 siderable numbers, mixed with other organisms. 



A considerable number of Eadiolaria are known from strata of 

 the secondary period, principally from rocks and coprolites of 

 Jurassic age, but a few also from the Chalk and Trias. A few 

 species have also been described from various rocks of Primary 

 age, ranging back from the Permian and Carboniferous to the 

 Devonian, Silurian, and Cambrian. These palaeozoic Eadiolaria 

 are all of an extremely simple and primitive structure, mostly 

 simple Spumellaria. 



The best known and most typical of all the fossil Eadiolarian 

 deposits are those of the Island of Barbados, which have been 

 studied in great detail by many observers, both geologists and 

 microscopists. Practically the whole island, which rises to a 

 height of 1147 feet and includes nearly 1G,000 acres, is composed 

 of various deep-sea deposits, such as globigerina oozes, clays, and 

 pure Eadiolarian oozes. Some of the latter contain as much 

 as 70 per cent, by volume of Eadiolarian skeletons. Haeckel 

 estimates the number of species in the Barbados deposits at 

 nearly five hundred, of which about 25 per cent, are living at 

 the present time. 



Our knowledge of the Eadiolaria has grown very rapidly since 

 1834, when the first Eadiolarian was described by Meyer. 



