94 AN INTRODUCTION TO THE STUDY OF FOSSILS 



dopodia (whence the name from Latin radiolus, a little ray). 

 There is usually present, in addition to the central capsule, 

 another skeleton, beautiful and delicate ; this is secreted by the 

 protoplasm, usually without, sometimes extending or originating 

 within the central capsule ; it consists of isolated spicules, bars 

 or latticework made of silica (Fig. 35), or rarely of acanthin, a 

 substance allied to chitin. When the silica is secreted it is 

 thrown down in the protoplasm all at once, apparently at the 

 supersaturation of the protoplasm with silica. 



The Radiolaria are marine organisms, floating at. the surface 

 of the ocean, or even at abyssal depths, and present in all 

 climates. In some deep parts of the Pacific and Indian oceans 

 these shells, falling after the death of the animal, accumulate 

 on the sea-bottom, forming a siliceous deposit, known as " ra- 

 diolarian ooze." The Radiolaria did not become important rock 

 builders until the Tertiary ; before this they occur only in scat- 

 tered siliceous cherts but from nearly all periods. The Bar- 

 bados earth, Miocene in age and covering large areas in the 

 Barbados Islands, resembles very closely the modern radio- 

 larian ooze and is probably a deep-sea deposit. Heliodiscus 

 and Astromma are two genera found here. 



In the " Barbados earth " the siliceous skeletons of radio- 

 larians still retain their originally amorphous condition. Usually 

 these skeletons are dissolved soon after burial in the rocks by 

 percolating w^aters containing alkali and may be redeposited 

 as chert. The chert nodules of the upper Cretaceous chalk are 

 probably largely due to these skeletons. 



Radiolaria occur from the pre-Cambrian to the present. 



CLASS B, MASTIGOPHORA 



Protozoa with a definite body outline, the body protected in 

 some species by a skeleton of cellulose or chitin, but usually 

 naked. Organs of locomotion and of food-capture in adults are 

 the flagella, — slender threads capable of whip-like lashing move- 

 ments (whence the name from Greek masHx, a whip, -|- pherein, 

 to bear). Euglena is an example. 



