PROTOZOA: RADIOLARIA. 57 



and it is not improbable that the large Silurian fossils known as 

 Receptaculites and Stromatopord should really be referred to this 

 order. In the Carboniferous rocks of Russia whole beds are 

 composed of a species of Fusulina. In the Secondary rocks 

 Foraminifera occur in great abundance, the widely-spread for- 

 mation known as the Chalk being crowded with these organ- 

 isms. Chalk itself, in fact, is almost entirely composed of the 

 cases of Foraminifera, some of which are identical with species 

 now existing. 



Fig. 5. Nummidites l&vigatus. Eocene. 



In the Tertiary rocks the Foraminifera attain their maximum 

 of development, both as regards the size and the number of 

 the forms which characterise them. The period of the Middle 

 Eocene is especially distinguished by a very widely spread and 

 easily recognised rock known as the Nummulitic Limestone, 

 so called from the abundance in it of a large coin-shaped Fora- 

 minifer termed the Nummulite (fig. 5). The Nummulitic Lime- 

 stone stretches from the west of Europe to the frontiers of 

 China ; but in some cases, in place of Nummulites proper, it 

 contains the remains of a mimetic form termed Orbitoides. 

 Upon the whole, Dr Carpenter concludes that " there is no 

 evidence of any fundamental modification or advance of the 

 foraminiferous type from the Palaeozoic period to the present 

 time." 



CHAPTER IV. 

 RADIOLARIA. 



THE order Radiolaria was founded by Miiller to include the 

 Polycystina, the Acanthometrina, and the Thalassicollida, to 

 which Dr Carpenter adds Actinophrys and its allies, chiefly on 

 account of the form of the pseudopodia. Here, however, the 

 term will be employed to designate the first three of these, and 



