160 Transactions of ihe 



beading of this diatom is well described by Mr. Keade {loe. cit.). 

 I have never been able to see the six black dots surrounding each 

 bead, mentioned by Dr. Pigott. 



No. XIII. Photograph of Coscinodiscus ; 1100 diameters. — This 

 photograph shows the spherical bosses well. It is apparently an 

 undeveloped frustule. On one side the hemispheres are quite a 

 httle distance apart. On the other side they are crowded together, 

 producing the spurious appearance of a hexagonal framework with 

 little spherical beads at the corners. 



No. XIV. Photograph of Pleurosigma angulatum, showing 

 beads; 3100 diameters. 



No. XV. The same ; 4500 diameters. — In these pictures the 

 illumination has been so managed as to give a considerable degree 

 of perspective to the markings. For comparison, I append a 

 print showing the appearances produced by more central illumina- 

 tion, viz. : — 



No. XVI. Photograph of Pleurosigma angulatum ; 3000 

 diameters. 



I conclude this paper with some brief remarks on two difficult 

 diatoms which have been the subject of much recent discussion, 

 viz. the Surirella gemma and the Am^hipleura peUucida. 



In the case of the Surirella gemma, Hartnack, whose original 

 description I have not yet seen, observed fine longitudinal striae in 

 addition to the fine transverse ones, previously known to exist 

 between the large transverse ribs. He supposed the true markings 

 to have the form of elongated hexagons. These are figured by 

 Dr. Carpenter* and by Frey.f The mean length of this diatom is 

 given in the ' Micrographic Dictionary ' (second edition) as -olot^ of 

 an inch. A careful examination of specimens mounted dry, has 

 satisfied me that Hartnack's interpretation is erroneous. The fine 

 striae are, I think, rows of minute hemispherical beads, and the 

 appearance of hexagons is the optical result of imperfect definition 

 or of unsuitable illumination. For photographing this object, 1 

 have selected a frustule of somewhat less than the medium size. It 

 measures ^ioth of an inch in length. Longitudinally, the fine 

 striae count at the rate of 72,000 to the inch. These striae are 

 resolved into beaded appearances which count laterally 84,000 

 to the inch. 



No. XVII. Photograph of Surirella gemma ; 3100 diameters. 

 (Portions of two frustules.) 



No. XVIII. Photograph of the same ; 1034 diameters. (Two 

 frustules and portions of a third.) 



Finally, with regard to the Amphipleura i^ellucida or Navicula 

 acus. The striae on this diatom were first described by Messrs. 



* ' The Microscope and its Revelations,' 4th edition, p. 182. 

 t ' Das Mikroskop,' 3rd edition, p. 40. 



