bottoms over which they live. In the great depths these 

 shells are absent, for they may be comi)letely dissolved while 

 they sink to the bottom, or shoi"tly after reaching it. 



Eadiolaria.— The Radiolariaare marine planktonie Proto- 

 zoa. They inhabit principalh^ the o})en sea, where they 

 occur at the surface or at various depths below it. In re- 

 gions 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 de])Osits 

 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 skeletons 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 com- 

 posed of radiolarian shells with some intermixture of sponge 

 spicules. The celebrated Barbados earth, a Tertiary deposit, 

 is likewise composed of radiolarian remains, to the exclusion 

 of almost every other substance. Fission, budding, and 

 spore formation constitute the methods of reproduction in 

 Radiolaria. The spores may be provided with flagella, con- 

 stituting "swarm spores," which, like their progenitors, 

 lead a planktonie existence. 



Sjxjughv.— The sponges are marine or fresh-water animals, 

 of a sedentary benthonic habit. In general only such species 

 as secrete a calcareous or siliceous skeleton — either continu- 

 ous or consisting of separate spicules — are capable of preser- 

 vation in a fossil state. The vertical distribution of marine 

 species ranges from the shore zone down to the greater 

 depths of the sea. Not infrequently species are found which 

 regularly undergo an exposure of several hours between 

 tides, though most littoral species occur below low-water 

 mark, or in tide pools from which the water is never drained. 

 Sponges will grow wherever a suitable surface for attach- 

 ment is found, the most usual substratum chosen being 

 cliffs, boulders, or the stems and "roots" of the larger algae. 

 In deeper and quieter water, the sandy and gravelly bottoms 



