W. M. BALE ON SOME OF THE DISCOID DIATOMS. 41 



Gyroptychus, Debya being an internal disc of A. undidatus, very 

 unlike the outer valves, and found by Van Heurck inside the 

 normal frustules. The A. jjellucidus Grunow, figured in Van 

 Heurck's synopsis, PI. 123, fig. 1, is, as will be obvious to any one 

 who compares it with the figure of A. Heliopelta in the same 

 plate, merely a valve of the latter with the border wanting and 

 the secondary reticulation undeveloped. In a genus-slide by 

 Thum I have several such valves, but for the most part they 

 retain a little more of the border, showing the origin of the 

 spines, and some of them also have the secondary markings more 

 or less distinctly indicated. 



In many marine gatherings from Port Phillip a form of 

 A. adriaticus is found in great profusion, of which a specimen is 

 figured in Schmidt's Atlas, PI. 153, fig. 14. It varies greatly in 

 the distinctness or otherwise of the secondary markings, and 

 especially in the presence or absence or fragmentary condition of 

 the narrow radial lines which in the typical A. adriaticus, as in 

 A . splendens, run outward from the umbilicus, or near it, to the 

 processes. In most slides a few specimens may be found with all 

 the areas alike, and a process on each, and it is this form which 

 has received the name of A. Molleri Grunow. 



Normally the areas are arched at the ends, as shown in Van 

 Heurck's figures, the secondary ones being shorter than the 

 primary, with a wide hyaline band outside them, but as in the 

 form called A. Molleri they are all primary areas, and conse- 

 quently of the same length, the hyaline band is reduced to a small 

 triangular area at the junction of every two compartments with 

 the margin. All the variations of marking which occur in the 

 normal valves are found equally in this form, and their specific 

 identity is obvious. In reality, this so-called A. Molleri is the 

 true A. adriaticus described by Grunow, his original figure 

 showing a valve with processes on all the areas, and exactly the 

 same marginal sculpture as described above. It is true A. Molleri 

 is supposed to be without the radial lines to the processes, but 

 Grunow recognised in his original description of A. adriaticus 

 that these lines might be present or not, in which he was 

 certainly correct. 



These radial lines, however (sometimes called pseudo-raphes), 

 appear to be considered by Van Heurck as distinguishing 

 A. adriaticus from A. vulgaris, though he admits a possible 

 exception in A. adriaticus var. pumila. In the common 

 Australian form, however, it is obvious that the presence of 



