MICROSCOPIC FOSSILS. 



000,000) forty thousand millions of distinct, organic forms. The species of Diato- 

 mjicese, of which nearly the whole mass is compacted, is the divided Gallionella, or 

 box-chain organisms; a kind of Diatomacese which has already been described. A 

 specimen from this slate is delineated in figure 78, magnified three hundred times. 

 Its natural length does not exceed one-sixth of the thickness of a human hair, and 

 the flinty shell of a single gallionella weighs only the one Fi e- 78. 



hundred and eighty-seven millionth part of a grain. The identity 

 of the fossil and living plantules is seen at a glance by comparing 

 the engravings in which they are respectively represented. In 

 Virginia, extensive beds of flinty marls have been discovered by Professor Rogers, 

 composed, in a great measure, of the shells of different species of marine organisms. 

 The towns of Richmond and Petersburg are built upon these strata, which vary in 

 thickness from twelve to twenty-Jive feet, and comprise tracts and districts of con 

 siderable extent. So full is this earth of microscopic fossil remains, that when a 

 little of it has been mixed with a drop of water, and the liquid has evaporated 

 from the glass slide, the smallest stain left upon the surface abounds with curious 

 vegetable structures, whose living types inhabit, to a great extent, the neighboring 

 seas. 



In figure 79 are shown two species of Navicula, which, with Fig. 79. 

 several others, have been recognized in the Richmond earth; but the 

 most exquisite structure here revealed is a beautiful saucer-shaped 

 shell, the surface of which is divided into hexagonal or six-sided 

 figures, like the cells of honey-comb. The protophyte to which it 

 belongs is called, from the appearance of the shell, the Coscinodiscus* or 

 sieve-like disk: there are several species of these organisms, whose 

 shells vary in size from one-hundredth to one-thousandth of an inch in 

 diameter. 



In figure 80 is displayed a portion of the circular shell of an elegant species 

 found in the Virginia marl, which has received the name of the Fi &- 80- 



Radiated coscinodiscus. It is shown very highly magnified, and 

 the rich and perfect arrangement of symmetrical forms here ex 

 hibited, is but a faithful copy of the wondrous original. These 

 beautiful fossil shells are not confined to the Richmond locality, 

 but have been discovered in the chalk marls of Zante and Oran; 

 and Col. Fremont likewise found them in Oregon, at the Riviere 

 Aux Chuttes. The various species of this protophyte exist in a 

 living state in the sea near Cuxhaven, at the mouth of the Elbe ; 

 and the radiated coscinodiscus has also been detected in the 

 waters of the Baltic, near Wismar. 



A like deposit of microscopic shells, fifteen feet thick, exists at 

 Andover, Ct., and Ehrenberg remarks, in his memoir on the 

 Microscopic life of North and South America, &quot;that similar beds 

 occur by the river Amazon, and in great extent from Virginia to 

 Labrador.&quot; 



From koskinon (Greek), a sieve. 



