34 



PRINCIPLES OF PALAEONTOLOGY. 



mention in this connection are the microscopic organisms which 

 are known to the naturalist as Polycystina. These little creatures 

 are the lowest possible grade of organization, very closely 

 related to the animals which we have previously spoken of as 

 Foraminifera, but differing in the fact that they secrete a shell 

 or skeleton composed of flint instead of lime. The Polycystina 

 occur abundantly in our present seas ; and their shells are present 

 in some numbers in the ooze which is found at great depths in 

 the Atlantic and Pacific oceans, being easily recognized by their 

 exquisite shape, their glassy transparency, the general presence 

 of longer or shorter spines, and the sieve-like perforations in 

 the walls. Both in Barbadoes and in the Nicobar islands occur 

 geological formations which are composed of the flinty skeletons 

 of these microscopic animals ; the deposit in the former locality 

 attaining a great thickness, and having been long known to 

 workers with the microscope under the name of " Barbadoes 

 earth" (fig. 15). 



Fig. 15. Shells of Polycystina from 

 'Barbadoes earth ;" greatly magnified. 

 (Original.) 



Fig. 16. Cases of Diatoms In the Rich- 

 mond " Infusorial earth ;" highly magni- 

 fied. (Original.) 



In addition to flint-producing animals, we have also the great 

 group of fresh-water and marine microscopic plants known as 

 Diatoms, which likewise secrete a siliceous skeleton, often of 

 great beauty. The skeletons of Diatoms are found abundantly 

 at the present day in lake-deposits, guano, the silt of estuaries, 

 and in mud which covers many parts of the sea-bottom; they 

 have been detected in strata of great age ; and in spite of their 

 microscopic dimensions, they have not uncommonly accumulated 

 to form deposits of great thickness, and of considerable super- 

 ficial extent. Thus the celebrated deposit of " tripoli " (" Polir- 



