ioo8 PRINCIPLES OF STRATIGRAPHY 



foraminiferal ooze in shallow water. But not only planktonic shells 

 but the benthonic species as well may form a pure accumulation 

 of foraminiferal shells, as has been the case in the chalk, in which 

 the planktonic species are practically wanting. (Walther-29:^75.) 

 Reproduction of the Foraminifera is carried on by fission, budding, 

 and spore formation. In the first two cases, the resulting part and 

 the buds have the characteristics of the parent, except its size, and 

 there are no special structures which serve for the greater distribu- 

 tion of the species. When spores are formed, these may be pro- 

 vided with a flagellum, whereupon the organisms pass through a 

 mero-planktonic stage. 



While the geographical distribution of the benthonic species is 

 very restricted, and influenced by the facies of the sea bottom, 

 the geographical distribution of the pelagic species is prevented 

 from being world-wide only by the changes in the temperature of 

 the water and by the ocean currents. The pelagic species are ex- 

 tremely abundant in tropical regions, and their shells form vast 

 accumulations on the sea-bottoms over which they live. In the 

 great depths these shells are absent, for they may be completely dis- 

 solved while they sink to the bottom, or shortly after reaching it. 



Radiolaria. The Radiolaria are marine planktonic Protozoa. 

 They inhabit principally the open sea, where they occur at the 

 surface or at various depths below it. In regions of terrigenous 

 sedimentation, or where an influx of fresh water occurs, these 

 animals are seldom met with. Hence their siliceous shells occur 

 in abundance only in the deposits found at a distance from shore, 

 and in deep water, where they may constitute as high as seventy 

 per cent, of the mass. The greatest abundance of radiolarian skele- 

 tons was found by the Challenger expedition at a depth between 

 2,000 and 4,475 fathoms — the greatest depth sounded. In many 

 places in the Pacific the bottom ooze is almost entirely composed 

 of radiolarian shells with some intermixture of sponge spicules. 

 The celebrated Barbados earth, a Tertiary deposit, is likewise com- 

 posed of radiolarian remains, to the exclusion of almost every other 

 organism. 



Fission, budding, and spore formation constitute the methods 

 of reproduction in Radiolaria. The spores may be provided with 

 flagella, constituting "swarm spores," which, like their progenitors, 

 lead a planktonic existence. 



II. PORIFERA. 



The sponges are marine or fresh-water animals, of a sedentary 

 benthonic habit. In general only such- species as secrete a calcare- 



