TJie Sfrnctiire of Diatoms. By Dr. J. II. L. Fldgel :A1 



detecting it, lu order to remain quite objective on this point with 

 regard to the sculpture of Phurosigma in general, I have made 

 photographs of a number of transverse sections, and amongst these 

 one of a central nodule of P. halticum (French specimen, 6, p. 480). 

 This shows without doubt that, as already proved by the cast 

 process, the nodule is a solid thickening of the wall projecting 

 inwards. 



(2) The Girdle-hand was formerly described by me (6, p. 480, 

 figs. 11 and 13) as a simple membrane. Pfitzer refuted this 

 correctly (19, p. 20). In my present sections made exactly 

 transverse to the median line, I see it sometimes single and some- 

 times double, probably due to the close proximity of the two plates. 

 If I gave no figures of this formerly, the reason was that in cases 

 where it appeared double I concluded that accidentally with the 

 imbedding in gum foreign matter adhered there, a supposition 

 which might be excused by the fact of using only sections made 

 through frustules lying pell-mell. 



(3) What I termed with P. halticum accessory rib (6, p. 481, 

 and fig. 13), namely, a small second rib-like edge on the one side 

 of the real median line, does not change position in all cases. For 

 example, if it lies on the right of the main rib in one valve it will, 

 as a rule, be seen in the other valve on the opposite side, that is, 

 on the left. Exceptionally it may be found in both valves on the 

 same side. Numerous experiments by crushing Pleurosigma valves 

 under heavy pressure have taught me that, contrary to the former 

 (6, p. 484) negative result, we can sometimes find fragments in 

 which the one membrane is isolated, that is to say, it appears 

 without any markings because the chamber-walls are rubbed off. 

 This, however, is only found in a very narrow edge-portion of such 

 fragments. Which membrane it is — whether the inner or the 

 outer— cannot be determined as a matter of course. Mistakes with 

 such fragments of uninjured valves, in which the markings are 

 indistinct because the chambers are filled up with a glutinous 

 substance, are avoided by convincing oneself of the much higher 

 refraction of the valve in this case, whilst the isolated membrane is 

 seen only very faintly. 



§ 2. Investigations hy others. — (1) Pfitzer in his essay (19, 

 p. 171) speaks of the sculpture of the cell-wall of Pleurosigma \ 

 but since nothing new is mentioned, I refer on this subject to the 

 General Remarks given in the third part of the present paper. 



(2) Mliller has also studied the question. I had sent him a 

 slide of Pleurosigma sections from the same gathering as the 

 French specimen described in my origioal paper, and soon after I 

 received from him two essays (14 and 15). As he was a novice in 

 section-making who had occupied himself with coarse objects 

 only, I treated his strange attacks with silence, in the hope 



