RADIOLARIA. 147 



desirous of fuller information, or anxious to study the fauna of any 

 particular Radiolarian deposit, will of necessity consult the works of 

 Ehrenberg, Rust, Haeckel, and others, who have specially devoted 

 themselves to the study of these minute organisms. 



As regards the past history of the great groups of the Radiolarians, 

 the Acantharia, in which the skeleton is composed of " acanthin," 

 are wholly unknown in the fossil condition. This is likewise the 

 case with the group of the Phaodaria — in which the skeleton is 

 composed of silicate of carbon — with the single exception of the 

 small group represented by Dictyocha and its allies, in which the 

 skeleton is purely siliceous. In Dictyocha (fig. 46, d), the skeleton 

 is composed of irregular bars of flint united into a loose network with 

 wide meshes. The genus begins in the Upper Chalk, and is repre- 

 sented in Tertiary deposits and in recent seas. 



With the above-noted exception, all the known fossil Radiolarians 

 belong to the group of the so-called " Polycystina" comprised in the 

 two divisions defined by Haeckel under the names Spumellaria and 

 Nassellaria. In all of these forms the skeleton is purely siliceous, 

 and it usually has the form of a porous latticed shell, the precise 

 shape of which varies in different types. 



Until recently, Radiolarians had only been detected in the fossil 

 condition in deposits of Kainozoic and Mesozoic Age ; and our 

 knowledge of Palaeozoic types of Radiolarians is still very incomplete. 

 According to Haeckel, however, Dr Riist (whose researches on this 

 subject are still unpublished) has discovered various types of Poly- 

 cystina, " of very simple form and primitive structure," in rocks as 

 old as the Silurian, or even the Cambrian. The remains of Radio- 

 larians have also been indicated as occurring in the Carboniferous 

 Limestone of England, but the true nature of these has not been 

 clearly established. 1 



Radiolarian remains have now been discovered in all the great 

 Mesozoic systems. From the Jurassic system, in particular, numer- 

 ous fossil Polycystina have been described by Zittel, Dunikowski, 

 and Riist. A noticeable Jurassic genus is Coinosphcera, which is 

 also found in the Chalk, in the Miocene marls of Barbados, and in 

 recent seas, and which has relationships with Collosphcera. Many of 

 the Jurassic Radiolarians occur in jasper, flint, or chert ; but they 

 are especially abundant in what have been termed " Radiolarian 

 quartzes." These are cryptocrystalline quartzites or hard siliceous 

 rocks, which are composed " for the most part of the closely com- 

 pacted shells of Spumellaria and Nassellaria " (Haeckel). The 



1 Considerable portions of the Carboniferous Limestone of Britain are occa- 

 sionally partially composed of the minute calcareous spheres which have been 

 described under the name of " Calcisphizrce.'" These enigmatical bodies are now 

 composed of carbonate of lime ; but it has been conjectured that they were origin- 

 ally siliceous, and that they are really referable to the Radiolaria. 



