FOSSIL INFUSORIA. 



fresh and salt water, are smaller still than the Foraminifera, 

 being only visible by the aid of microscopes of great magnifying 

 power. There are innumerable species of them, which are fur- 

 nished with siliceous shells, and which, consequently, are accu- 

 mulated at the bottom of the waters with the contemporaneous 

 microscopic plants. Now, although these beings are so minute 

 that forty thousand millions of them would not fill a space of 



Fig. 100. 



Fig. 101. 



Fig. 102. 



Fig. 103. 



Fig. 100. a. Nodosaria limbata. Fig. 102. a. Flabellaria rugosa. 



6. Internal arrangement of b. Side view, to show the flat 



the cells. 

 101. a. Marginulina trilobata. 



b. The last cell seen from 



above. 



c. The internal arrangement 



of the cells. 



103. a. Textularia turris. 



b. Internal arrangement of 

 the alternate cells. 



more than a cubic inch, M. Ehrenberg has demonstrated that 

 their accumulation in certain parts of the earth's crust has pro- 



Fig. 104. 



Fig. 105. 



Fig. 106. 



Fig. 104. Rotulina Voltzii. 

 ,, 105. a. Cristellaria rotula. 



Fig. 105. b. Edge view, to show the flatness. 

 ,, 106. Orbiculina numismalis. 



duced strata several yards in thickness, and of vast extent ; and 

 that in many other cases strata not less extensive are formed by 

 their combination with other conchiferous animalcules. They 

 constitute almost exclusively the polishing slate of Bilin, in 

 Bohemia, which occupies a surface of great extent, probably the 

 site of an ancient lake, and forms a stratum fourteen feet 

 in thickness, composed of the mineralised shields of these 

 animalcules. " The diameter of a single one of these creatures," 

 says M. Ehrenberg, "amounts upon an average, and in the 

 greatest part, to the 3500th of an inch, which equals | of the 

 thickness of a human hair, reckoning its average size at the 

 570th of an inch. The globule of the human blood, considered at 

 the 3600th of an inch, is not much smaller. The blood globules 



K 2 131 



