DIATOMAC&E 62 1 



as there are other reasons for believing to be occasionally formed, 

 would account for the presence of Uiatomaoew in volcanic ashes 

 and pumice which was discovered by Professor Ehrenberg. It is 

 remarked by Sir J. D. Hooker that the universal presence of this 

 microscopic vegetation throughout the South Polar Ocean is a most 

 important feature, since there is a marked deficiency in this 

 region of higher forms of vegetation ; and were it not for them, there 

 would neither be food for aquatic animals, nor (if it were possible 

 for these to maintain themselves by preying on one another) could 

 the ocean waters be purified of the carbonic acid which animal 

 respiration and decompositionXypuld be continually imparting to them. 



FIG. 468. Fossil Diatomaceae, &c., from Mourne Mountains, Ireland : a, a, a, 

 Gaillonella (Meh>sira)procera and G. granulata ; d, d, rf, G.biseriata (side view) ; 

 b, b, SurireUa plica t a ; c, S. craticula; k, S. caledonica; e, GompJwnema 

 gracile; /, Cocconema fiiaidhun ; g, Tabellaria vulgaris', li, Pinnularia 

 dactylus ; /, P. nobilis ; /, Sij)iedra ulna. 



It is interesting to observe that some species of marine diatoms are 

 found through every degree of latitude between Spitzbergen and 

 Victoria Land, whilst others seem limited to particular regions. One 

 of the most singular instances of the preservation of diatomaceous 

 forms is their existence in guano, into which they must have passed 

 from the intestinal canals of the birds of whose accumulated excre- 

 ment that substance is composed, those birds having received them, it 

 is probable, from shell-fish, to which these minute organisms serve 

 as ordinary food. 



The indestructible nature of the silicified casings of Diatomacece 

 has also served to perpetuate their presence in numerous localities 

 from which their living forms have long since disappeared ; for the 



