MR. A. W. WATERS ON FOSSIL LITHOTHAMNIA. 249 



far the most important and numerous species can scarcely 

 be distinguished by any other method than by the form 

 and relative size of the cells, which can only be made out 

 by transparent sections." We must be very careful how 

 we accept this ; for in a piece I prepared, from Torbole, on 

 the Lake of Garda, a drawing of which is shown, we find 

 the same plant producing distinctly three sizes of cells — at 

 first the cells being large ; then at right angles to these 

 cells grow out others much smaller, the same process 

 being repeated a second time. The three sets of cells are 

 in the proportion 5:3:2. In any branch we take we 

 shall find the centre cells, which are arranged along the 

 long axis, much larger than those at the sides, which turn 

 outwards. 



According to RosanofF, the form of the young is always 

 orbicular. The germinal cell divides into two ; these again 

 divide; then the cells grow concentrically round these, 

 usually soon losing the orbicularity, and growing princi- 

 pally in one direction by occasional radial division of the 

 cells. The cells, when cut through in a branch at right 

 angles to the direction of growth, are circular. 



Gumbel gives the average of various analyses of the Lit ho - 

 thamnium nodosum from Vienna and Monte Mario : — 



Lime 47 #I 4 



Magnesia 2*66 



• Alumina, iron, and oxide of manganese 2*55 



Phosphoric acid 006 



Carbonic acid 40*06 



Insoluble in acid 4^96 



Water and loss 2*57 



IOO'OO 



You will notice, beside the large amount of carbonate of 

 lime, the large proportion of magnesia, representing 5§ per 

 cent, of carbonate of magnesia. 



Gumbel points out the important bearing the large 

 amount of magnesia which plants and animals can take 



