FOSSIL INFUSORIA. 



55 



Fig. 79. 



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 Fj(r ?8 



of a single animalcule weighs only the one hundred and eighty - 

 seven millionth part of a grain. The identity of the fossil and 

 living animalcules is seen at a glance by comparing the engra- 

 vings in which they are respectively represented. In Virginia, extensive beds of 

 flinty marls have been discovered by Prof. Rogers, composed, in a great measure, of 

 the shells of different species of marine animalcules. The towns of Richmond and 

 Petersburg are built upon these strata, which vary in thickness from twelve to twen- 

 ty-five feet, and comprise tracts and districts of considerable 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 Infusorial structures, whose living 

 types inhabit, to a great extent, the neighboring seas. 



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

 several others, have been recognised 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 a honey-comb. The animalcule to 

 which it belongs is called, from the appearance of its shell, the 

 Coscinodiscus* or sieve-like disk : there are several species of these 

 Infusoria, whose shells vary in size from one-hundredth to one-thou- 

 sandth 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 Fig 80 



the Radiated coscinodiscus. It is shown very highly magnified, 

 and the rich and perfect arrangement of symmetrical forms 

 here exhibited, 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 

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

 Riviere Aux Chuttes. The various species of this animalcule 

 exist in a living state in the sea near Cuxhaven, at the mouth 

 of the Elbe ; and the Radiated coscinodiscus has also been de- 

 tected in the waters of the Baltic, near Wismar. 



A like deposit of Infusorial shells, fifteen feet thick, exists at An- 

 dover, Ct, and Ehrenberg remarks, in his memoir on the Micro- 

 scopic life of North and South America, " that similar beds oc- 

 cur by the river Amazon, and in great extent from Virginia 

 to Labrador." 



In Sweden and Lapland, a white, mealy earth is found distributed in layers, 

 sometimes thirty feet in thickness. It is wholly composed of . the shells of ani- 



* From kosMnon, (Greek) a sieve. 



