PROTOPHYTES. 93 



chrome, with a firm external covering. In the Diatoms, 

 however, this envelope is consolidated by silex, or flinty 

 matter, sometimes also by iron. The silicious envelope 

 of each " frustule," or cell, is covered with most elabo- 

 rate and beautiful marking, and consists of two valves, 

 or plates, closely applied to each other, like the valves of 

 a Mussel, along a suture, or line of contact. If the valve 

 is hemispherical, the cavity will be globular; if a segment 

 of a sphere, the cavity will be lenticular. Sometimes 

 the central part is flattened, and the sides turned up, so 

 that the valve resembles the cover of a pill-box, in which 

 case the cavity will be cylindrical. Then, again, the 

 valve may be square, triangular, round, heart-shaped, 

 boat-shaped, etc. In many species of Diatoms the 

 markings are so minute that they can only be made out 

 with the highest powers of the microscope ; in others a 

 very moderate power suffices to exhibit the lines and 

 dots in patterns which rival the most elaborate works of 

 art. (Figs. 32, 33, 34.) 



In the living state Diatoms are found abundantly in 

 every pond, rivulet, ocean, and rock-pool. In some 

 parts of the world they form immense deposits. 



A mud bank in Victoria Land, 400 miles long and 120 

 broad, is composed of silicious valves of Diatoms. In 

 Sweden and Norway, under the name of bergh-mehl, 

 they are used for mixing with flour for bread in seasons 

 of scarcity. Under the cities of Richmond and Peters- 

 burgh, Virginia, is a deposit twenty feet thick, while the 

 polishing slate of Bilin contains Diatoms so small that in 

 a single cubic inch 4O,ooo,ooo,cxx),ooo (forty trillions) are 

 found. 



