ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



50- 



Diatoms in thin Rock Sections * — W. Prinz, in the first place, 

 refers to the contrary opinions that have been held as to the sculp- 

 turings of diatom valves, some considering them elevations, others 

 depressions, and others perforations, the phenomena of interference 

 and diffraction serving to conceal the true structure. Photomicro- 

 graphy was not found a certain guide, as Dr. Woodward's photographs 

 showed elevations, and those of C. Giinther f openings in P. angidatum. 



The author recently received some pieces of a diatomiferous rock 

 from Nykjobing, Jutland, the examination of which has enabled him, 

 he considers, to establish the true character of the " markings." The 

 rock is almost as hard as limestone, and exhibits bedding due to the 

 superposition of very thin laminae. Slices at right angles to the 

 bedding planes gave sections of the diatoms exhibiting the form 

 of the sculpturing on the surface of the valves. In order to render 

 the structure of the sections more clear, and to remove any impurities, 

 it was found advisable to treat the prepared films of rock with hydro- 

 chloric acid before mounting them in balsam. In thin sections, 

 properly prepared, the diatoms will be found cut in every plane. 



The three species described by M, Prinz are Coscinodiscus oculus 

 Iriclis, Trinacria Eegia Heib., and Coscinodiscus excentricus. These he 

 examined, in the first instance, in a free condition and with the frus- 

 tules entire ; but it is unnecessary to reproduce the descriptions of 

 that part of the examination. 



Coscinodiscus oculus Iridis is covered with a reticulated design, the 

 result of the superposition of two layers ; the upper formed of hexa- 

 gonal cavities resembling the alveoli in a honeycomb, and the lower 

 composed of small areola, convex according to some, concave accord- 



FiG. 114. 



Fig. 115. 



ing to others, placed each in the centre of one of the hexagonal 

 depressions. 



A very oblique section, cutting successively the two layers com- 

 posing the valve, exhibits their structure separately. Fig. 114 shows 

 one of these sections, and Fig. 115 the details much enlarged. In the 

 latter figure the hexagonal alveoli (more and more cut away) are seen 

 to disappear completely, and leave the lower layer with its areola 

 bare. The margin of the section clearly shows that these areolae are 



* Ann. Soe. Belg. Micr.— Mem., vii. (1S80) 12 pp. (1 pi.), 

 t See tliis Journa], iii. (1880) p. 891. 



