RADIOLARIA. 1 49 



to Haeckel, the number of species of Radiolarians in the Barbados 

 earth is not less than four hundred, and is probably more than five 

 hundred. This eminent authority regards the Barbados earth as a 

 deep-sea deposit, and states that very many of the Barbados Radio- 

 larians " are to-day extant and unchanged in the Radiolarian ooze 

 of the deep Pacific Ocean." 



Very similar to the Barbados earth is the Polycystine marl or 

 " tripoli " of Sicily, Calabria, Greece, and Northern Africa, the age 

 of which is also Miocene. Another deposit of the same nature is 

 the tripoli or Radiolarian clay of the Nicobar Islands, which rises 

 to elevations of about 2000 feet above the level of the sea, and is 

 probably of Miocene or Oligocene age. 



Of the more common Tertiary genera of Radiolarians, Haliomma 

 (fig. 46, c), Heliodiscus, Adinomma, and Didymocyrtis belong to a 

 group of forms in which the skeleton consists of two, three, or 

 more porous spherical shells, included concentrically within one 

 another, the smaller within the larger, and united by radial bars. 

 In other cases, as in the genera Podocyrtis (fig. 46, a), Eticyrtidium 

 (fig. 45, c), Lychnoccuiium (fig. 46, /"), and Didyomitra, the skele- 

 ton has the form of a latticed shell, which may be undivided, or is 

 partially marked off into two or more compartments by transverse 

 constrictions. The two poles of the shell in these cases are quite 

 unlike each other, and the membranous capsule of the living animal 

 is included within the closed apical pole. In other cases, again, the 

 skeleton consists of a flat, or lenticular and biconvex plate, which is 

 sometimes double, and has a more or less complex internal structure. 

 This type of skeleton is found in such genera as Astromma, Trema- 

 todiscus, Rhopalastrum, Stepha?iastru7n, and Stylodictya (fig. 45, a). 



LITERATURE. 



FORAMINIFERA. 



i. "Introduction to the Study of the Foraminifera." 'Ray Society.' 

 1862. W. B. Carpenter. 



2. " Report on the Foraminifera." ' Rep. on the Sci. Results of the 



Voyage of H.M.S. Challenger,' vol. ix. 1884. Henry B. Brady. 



3. "Carboniferous and Permian Foraminifera" (with a general Intro- 



duction). 'Monographs of the Palaeontographical Society,' 1876. 

 H. B. Brady. 



4. " Handbuch der Palaeontologie," vol. i. pp. 61-114, 1876. Zittel. 



5. " Mikrogeologie." Ehrenberg. 1854. 



6. " Foraminiferes Fossiles du Bassin Tertiaire de Vienne." D'Or- 



bigny. 



7. " Entwurf einer systematischen Zusammenstellung der Foramini- 



feren." ' Sitzungsber. d. k. Akad. Wiss. Wien,' 1861. Reuss. 



8. "Monograph of the Foraminifera of the Crag," 1868. Rupert Jones, 



Parker, and Brady. 



