the Valves of Pleurosigma and other Diatoms. 859 



Mr. Hall* is fully established to the author's (Professor Schultze's) 

 satisfaction by some photographic representations procured by 

 the aid of Hsulnack's combination. According to these figures, 

 the Hues in each set are not continuous in a straight direction, but 

 are bent at short intervals, at an angle of 120°. These bends 

 are, however, so close together as to be imperceptible with the 

 power usually employed in the examination of P. angulatum, 

 that is, with one of from 500 to 800 diameters. It is especially 

 by oblique light (under which only it is generally the case that 

 the sets of ridges appear as continuous striae) that the i/Zu-. 

 sion (?) becomes perfect that we are beholding sets of lines 

 running in a perfectly direct course ; whilst observations with 

 direct illumination, provided that the lenses have sufficient de- 

 fining power, discloses the true state of things" (?). 



It will be seen from the above extract (which I have deemed it 

 unavoidable to quote in detail) that Prof. Schultze bases his rea- 

 soning on a fundamental error, namely, the presence of only three 

 sets of decussating lines in Pleurosigma angulatum and its allies, 

 whereas they invariably present four : that is to say, two series 

 traversing the valve diagonally, so as to leave lozenge-shaped 

 spaces between each of these intersections ; and two series, one 

 of which is parallel to the longitudinal axis of the valve, whilst 

 the other crosses this at right angles. In the quincuncially 

 marked group the diagonal lines alone maintain one uniform 

 plane, and hence are invariably the most distinct; whilst the 

 rectangular lines, which again intersect the axes of the lozenge- 

 shaped spaces, follow the alternately ascending and descending 

 outline of the pyramidal projections by which these spaces are 

 occupied, and hence constitute a series of vertical zigzags, which 

 causes them to be defined with much greater difficulty. 



In the rectangularly striated Pleurosigmata, of which P. balti- 

 cum is the type, the rectangular sets of hues are the most dis- 

 tinctly marked, for a similar reason. They are not only on the 

 same plane, however, as the diagonal lines are in the quincun- 

 cial group, but, as I have recently satisfied myself, they consti- 

 tute the actual surface of the valve, whilst the spaces bounded 

 by them are pyramidal depressions. In the one case, the frag- 

 ments of a comminuted valve, when \iewed in section, show that 

 the apices of the pyramidal elevations are the most prominent 

 points on the external surface ; in the other, that the ridges, or, 

 in other words, the rectangular striae, are the most prominent. 



As stated by me in my former paper on this subject (Annals 

 and Magazine of Natural History, February 1860), analogy has 



* Quarterly Journal of Microscopical Science, vol. iv. pi. 13. fig. 2; and 

 Micrographic Dictionary, 1856, pi. 4/. figs. 41 and 48 ; the former of Which 

 ia " copied from a photograph by Mr. Wenham." 



