the Valves of Pleurosigma and other Diatoms. 357 



of those lines. This view of the structure I shall attempt to 

 prove to demonstration in a later portion of the present paper. 

 But it is necessary to state in this place that, whatever parts of 

 the valvular surfaces may have been represented as "promi- 

 nences " in Mr. Wenhanr's most interesting electrotype casts, in 

 his later observations on the photographic representation of 

 these structures no facts have been advanced to indicate that 

 the portions supposed to represent those in relief on the photo- 

 graphs are not identical with those actually in relief on the 

 casts ; and hence it is reasonable to infer that the parts which 

 really are convertible into " dots," &c. (that is the spaces be- 

 tween the intersections of the lines, and not the lines or striae 

 themselves), constitute the seat of the elevations as presented on 

 the casts *. But here, again, we are met with a further difficulty, 

 if my views be correct ; for whereas I am of opinion that the in- 

 tervening spaces in P. angulatum and its allies exhibit elevations, 

 I think it probable that in P. hippocampus and the rectangu- 

 larly marked series the intervening spaces between the lines, as 

 seen on the external surface of the valves, are occupied by de- 

 pressions. 



Welcker's system for determining whether minute points of 

 structure seen under the microscope consist of elevations or 

 depressions on the general surface, although of service where 

 these markings are scattered so as to leave well-defined intervals 

 between them, becomes practically useless in testing the confi- 

 guration of such objects as the Pleurosigmata, on the valves of 

 which the alternate elevations and depressions are in immediate 

 contiguity. Schultze fully recognizes this difficulty, but endea- 

 vours to explain it away, as I conceive, somewhat unsuccessfully. 

 He alludes to the appearance presented by the larger of the 

 artificially prepared conical papillae of fluoride of silicium, and 

 inclines to the belief that the successively seen "concentric 

 rings" are attributable to these papillae being "composed of 

 superimposed laminae, gradually diminishing in size." 



The same appearance of concentric rings is observable, how- 

 ever, when any translucent uniformly curved projection or de- 

 pression is viewed under the microscope, the illusory effect being 

 much more marked where the penetrating power of the objective 

 employed is limited. The truth of this statement may readily 

 be put to the test by examining the valvular surface of some of 

 the Melosira (M. Westii, for example), in which a continuously 



* For reasons which will presently become manifest, I would mention 

 that this remark applies only to the rectangularly lined series of the Pleuro- 

 sigmata. Mr. Wenham, in his paper on the electrotype experiments, makes 

 special reference to P. balticum and P. hippocampus, but to none of the 

 diagonally lined series — a fact of no little importance, for similar reasons. 



