DIATOM ACEiE. 423 



often seems as if the appearance were due to pyramidal 

 elevations, with hexagonal bases, standing in regular rows, 

 exactly like those observed in the siliceous vesicles above 

 mentioned. 



"In any case, it seemed likely, since the marking just 

 noticed is essentially alike in many different species of 

 diatoms, that its ultimate causes were to be sought, less, 

 perhaps, in any organic formative process, than in the 

 deposition of silex under the same laws as those by which 

 its deposition in the other case is regulated. Were this ulti- 

 mate cause shown to be crystallisation the question would 

 be solved. But that the secretion of amorphous silex in 

 diatoms, as in these pellicles, is undeniable, on account of 

 their specific gravity being about 2*2, the specific gravity 

 of crystallised silex being 2'Q.^^ 



But further investigation rendered the correctness of 

 this assumption (that the markings are due to crystallisa- 

 tion) doubtful in the highest degree, and it soon became 

 quite certain that neither in the artificial siliceous pellicles 

 nor in the diatom valves are the peculiar forms essentially 

 due to a crystalline structure. 



Mr. Charles Stodder, of Boston, says, " I have reasons 

 for thinking that neither party has the true explanation 

 of the structure. My opinion is that the exterior of the 

 shell is smooth, or nearly so, and that the borders of the 

 hexagons, or other shaped areolae, and costse of the Cos- 

 tate forms, are internal projections from the outer plate, 

 as on the under side of the leaf of the Victoria Regia, in- 

 tended to give strength to the cell with the smallest 

 quantity of material. This will explain the trace of the 

 hexagons seen on the inner plate of Heliopelta, as only 

 the projecting wall of the areolae would come in contact 

 with the inner plate. Dr. Griffiths reasoned that the 

 areolae were depressions, because they were the thinnest 

 parts of the shell ; the facts are correct, but the inference 

 may not be, as there is another explanation of the pheno- 

 mena." See directions for the illumination of test-objects, 

 page 174. 



Movements of Diatoms. — The researches of Professor 

 Max Schultze, of Bonn, published 1865, appear to throw 

 some light on the vexed question of the movements of the 



