EDWARD M. NELSON ON DIATOM STRUCTURE. 85 



(Fig. 6, PI. 8). Up to this point we have not had any trace of the 

 finely perforated central membrane, but now it begins to appear 

 round those intercostal spots which have divided. Fig. 5, PI. 8 

 shows this stage, and the suggestion I offer is that the minute per- 

 forations 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 activity is confined solely to the intercostal dots ; 

 in short, it is from the intercostal point that the whole secondary 

 structure originates. This brings us to what has been considered 

 as the mature Asteromphalus pattern, a number of large perfora- 

 tions, more or less similar in size, surrounding a finely perforated 

 sieve-like membrane, the whole forming a cap to the primary 

 polygonal structure. The discovery of the new tertiary structure 

 would seem to show that the ordinary Asteromphcclus is not the 

 terminus a quo of this diatom, but that after a time 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 w^hole of the valve. Diatoms of this form do 

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

 common Asteroinj^halus is merely a stage in their evolution? 



The following is an enumeration of the steps in the evolution : — • 



1st. A small equilateral triangle is formed at the intercostal 

 junction of the polygonal cells (Fig. 2). 



2nd. The angles of the triangle become blunted. 



3rd. The blunted end becomes notched (Fig. 4). 



4th. The notches deepen, and eventually becoming circular, 

 form a perforation at each intercostal angle. At the same time 

 the sides of the triangles form a larger perforation between them. 

 These larger perforations are situated at the bisection of the 

 sides of the hexagons (Fig. 9). 



5th. The peripheral perforations situated at the intercostal 

 angles break into two, and by repeated subdivisions form the 

 central finely perforated membrane (Figs. 6 and 7). 



6th. The peripheral perforations, w^hen the central membrane is 

 complete, become more or less of a uniform size, and then break 

 up into tertiaries (Fig. 8). 



