6 THE AMERICAN MONTHLY [Jan 



which are at the intercostal angles, are not so large as 

 those at the bisection of the sides of the hexagon. At 

 this point, then, we have twelve perforations round each 

 hexagon, six large ones at the bisection of its sides, and 

 six small ones at the intercostal angles. At the next step 

 the small intercostal perforations become elongat»^d,then 

 notched, and eventually divide into two. The microsco- 

 pical resolution of these is precisely similar to the split- 

 ting of close double stars with a telescope (Fig. 6). Up 

 to this point we have not had any trace of the finely per- 

 forated central membrane, but now it begins to appear 

 round those intercostal spots which have divided. Fig. 

 5 shows this stage, and the suggestion I offer is that the 

 minute perforations in the central portion are formed by 

 the breaking up of the six peripheral intercostal dots. 

 Thus far in no case has the large dot at the bisection of 

 the side of the hexagon been seen to break up ) the ac- 

 tivity is confined solely to the intercostal dots; in short, 

 it is from the intercostal point that the wlidl^ secondary 

 structure originates. This brings us to what has been 

 considered as the mature Asteromphalus pattern, a num- 

 ber of large perforations, more or less similar in size, 

 surrounding a finely perforated sieve-like membrane, the 

 whoie forming a cap to the primary polygonal structure. 

 The discovery of the new tertiary structure would seem 

 to show that the ordinary Asteromphalus is not the ter- 

 minus a quo of this diatom, but that after a lime the 

 whole of the peripheral dots, those at the bisections of 

 the sides as well as those at the intercostal angles, break 

 up and form an extended perforated membrane. By this 

 means the intercostal silex becomes so reduced that the 

 operation need only be repeated two or three times, when 

 a uniformly perforated membrane will be formed over 

 the whole of the valve. Diatoms of this form do exist : 

 may they not be regarded as the final forms, while the 

 common Asteromphalus is merely a stage in the evolution. 



