150 THE SEA. 



sand from the Adriatic, and D'Orbigny estimated the 

 number in a pound of sand from the Caribbean Sea 

 at no less than 3,849,000, nearly four millions of 

 individual animals ! 



Mr Macdonald observes, that the spicula of Sponges 

 and Asteroid Polypes, and the minute embryonic 

 shells of Gastropoda, Pteropoda, and Conchifera, are 

 usually found with the Foraminifera in the sound- 

 ings which he has examined. The pelagic shells, or 

 those which during life rove freely through the sea, 

 descend into the profound recesses after death by 

 their own gravitation ; but the others are washed off 

 from every coast and reef; millions of organic and 

 almost indestructible forms thus combining every day 

 and hour to enrich the dark and solitary bed of the 

 ocean, and to smoothen its rugged floor. The 

 muddy bottom of the sea outside the Capes of Port 

 Jackson is nearly altogether composed of such mate- 

 rials, as is that which fringes a considerable portion 

 of the coast of North America, and other vast regions. 



A few particulars of the life-history of these atoms, 

 which play a part so important in the physical eco- 

 nomy of the earth, may not be unacceptable to my 

 readers. The older conchologists were acquainted 

 with a few shells of microscopic minuteness, some of 

 which closely resembled in form that of the Nautilus, 

 and, like it, were found to be divided into successive 

 chambers. For a long time these tiny forms were 

 considered as Mollusca, and belonging to that highest 

 type of structure, which includes the Nautilus and 



