216 THE OCEAN 



least ; many species of pteropods, heteropods, 

 gasteropods, foraminifera and calcareous algae 

 (coccospheres and rhabdospheres) flourish 

 in the surface waters of tropical regions, and 

 the largest and thickest- shelled specimens are 

 found in the regions of the equatorial calms. 



During the " Challenger " Expedition an 

 attempt was made by the writer to estimate 

 the quantity of calcium carbonate in this form 

 in tropical surface waters. A tow-net with 

 an opening a foot in diameter was dragged 

 for as nearly as possible half a mile through 

 the water, and the shells collected were boiled 

 in caustic potash, washed, dried and weighed, 

 the mean of four experiments giving 2-545 

 grams. If the calcareous organisms were as 

 abundant at all depths down to 100 fathoms 

 as they were in the track followed by the 

 tow-net, there would be sixteen tons of calcium 

 carbonate in this form in a mass of tropical 

 oceanic water one square mile in area by 100 

 fathoms in depth. 



Proceeding polewards from the tropics these 

 calcareous organisms become smaller and 

 many varieties die out, until in the cold 

 surface waters of the Arctic and Antarctic 

 regions only one or two thin- shelled species 

 of pteropods, and one or two dwarfed species 

 of foraminifera, occur. In like manner much 

 less calcium carbonate is secreted in the cold 



