672 Transactions of the Society. 



to which part the portion belongs which is under examination. In 

 order not to err on this point I have kept to the pointed ends in 

 the investigation of the valves, and which exhibit in the section 

 comparatively very small rings. Photograph 19 illustrates a 

 section through two frustules and through one connecting end 

 being the isthmus proper. The small ring, without doubt the 

 valve, shows on the inner side distinct projections, that is to say, 

 wall-thicknesses apparently vanishing like network produce the 

 cell-figure in the surface view. The membrane at the non- 

 thickened end, that is to say, at the lumen of the pseudo-cells, 

 is of extreme thinness. The immediately preceding much finer 

 section corroborates still more what I say. The thickness of the 

 wall is about • 3 yti, and in the net projections 1 " 2 /*, we thus 

 obtain an image of simple inner cell-envelope thickenings in a 

 manner leaving nothing to be desired, and which has not the 

 slightest similarity to Triceratium. The cell- wall of the middle 

 girdle differs inasmuch as the thickening lines producing the 

 markings are undoubtedly on the outer side. The wall-thickness 

 is so extraordinarily small that with a magnification of 1000 it 

 aj)pears only as a mere line. The net-lines are also very flat, 

 about 0'7 yu. in height. "With reference to the girdle-band 

 one can speak with full conviction of a surface-sculpture, whilst 

 with the valves one must say inner surface-sculpture. Here 

 may be added that Isthmia has a large cell-nucleus lying in the 

 inner granular protoplasm, and which was touched by me several 

 times in my sections. It is a spherical transparent vesicle of 16 yc* 

 diameter, having a spherical nucleolus of 4 ' 5 yu. in diameter. The 

 result obtained from surface views of dry imbedded Isthmia does 

 not at all agree with that obtained with the transverse section 

 images ; the thickening lines appear like strong refracting masses, 

 and were looked upon as such by the earhest investigators 

 (Ehrenberg, Kiitzing, and others). It was not at all to be expected 

 from an a priori examination that the sculpture of this species 

 would appear so totally different from Triceratium. I shall not be 

 expected to enter further into the researches of Slack (25), according 

 to whom the membrane consists of small spheres. 



8. Achnanthes. 



If an obstinate defender of the opinion that diatom sculpture 

 consists of inner cell-wall thickenings, wishes to secure an object 

 substantiating his view, I can very strongly recommend to him the 

 large forms of Achnanthes. After having occupied myself inland 

 for years with fresh-water diatoms, on meeting with the marine 

 Achnanthes I believed I had found the long-searched-for proof. 

 Each surface view under a good Microscope shows clearly the 



