680 Transactions of the Society. 



Btructure from the one at the otlier corner seems probable at the 

 first glance, but nothing definite on that point could be made out. 

 It seems to me that the spine is the outdrawn end of a general 

 gelatinous cover secreted by the entire valve-surface. The cell- 

 nucleus of Achnanthes, fig. 46, is a small spherical vesicle of 4 yu- 

 in diameter with nucleolus of 1 /x. 



9. Synedra. 



With Achnanthes I have given numerous transverse sections of 

 Synedra Gallionii Ehrenb., often and everywhere found in Kiel 

 harbour, fig. 58. Beyond its general form very little can be 

 deduced, especially as we are left in the dark as to how the in- 

 significant transverse striae are caused on the edges of the square. 



III. Kesults and Genebal Eemaeks. 



The detailed researches given in the preceding chapters shall 

 only be mentioned here in so far as they relate to the sculpture of 

 the cell-envelope, whilst I must refer to the above for the facts in 

 connection with the girdle-bands, fission, &c. These researches 

 comprise in all, seventeen varieties of diatoms (Pinnularia major, 

 viridis, and Crdbro ; Navicula Lyra ; Pleurosigma halticum, 

 angulatum, Scalprum ; Surirella hiseriata ; Triceratium Favus ; 

 Coscinodiscus radiatus, ocidus iridis, centralis, and concinnus; 

 Isthmia enervis; Eupodiscus Argus; Achnanthes hrevipes, and 

 Synedra Gallionii), which have all been examined by the section- 

 method, also to some extent by other methods. The result is that 

 the marking of the diatom coatings has its origin in various forms 

 of the wall-thickness and in the cavity formation within the 

 membrane. The results can be grouped as follows : — 



The marking is caused : — 



(1) by the sharply projecting wall-thicknesses. 

 a, on the inner surface of the membrane : 



Achnanthes = transverse striae, Isthmia = valves, 

 probably also Grammatophora, Epithemia, and 

 others.t 

 h, on the outer surface of the membrane : 



Isthmia = girdle-band. 



(2) by developed chambers within the membrane, and 

 a, with distinctly observable openings. 



* Which are on the outer surface of the cell, whilst they 

 are closed inwards (in a certain degree transition from 

 type 1 &) : 



Triceratium, Coscinodiscus radiatus, and possibly a 

 few other varieties. 



t Weiss, 28, p. 9 ; Miiller, 16. 



