the Valves of Pleurosigma and other Diatoms. 363 ' 



are four sets of lines, and not three only, as stated by Pro- 

 fessor Schultze. This is a material point; for there are only 

 three sets necessary in the formation of a series of hexagons. 

 And, lastly, I have to repeat that, whereas a change in the 

 series which happens to be most distinct (owing to the direction 

 in which the light comes) takes place four times during a com- 

 plete revolution of the stage (that is to say, once in every 90°, 

 when P. angulation is examined), the change would necessarily 

 take place six times, or once in every 60°, were the hexagonal 

 figure the true one. 



Lastly, it is obvious that, if the stria? or lines are those parts 

 of the valve which are " resolvable into dots '* or hexagons, two 

 of each of these lines ought to enter into the formation of op- 

 posite sides of each hexagon. In Schultze's figure* two of each 

 series are actually represented as entering into the formation of 

 each hexagon. Hence, for every three hexagons there ought to be 

 four lines. But it is not the lines which, under the circum- 

 stances described, become resolvable into dots or hexagons, but 

 the spaces between the intersections of those lines ; and hence it 

 will be found that the number of " dots " occupying a given 

 number of divisions of the micrometer does not coincide with 

 the number of lines ascertained as occurring in a like space. 



There is one point, however, on which I have found it neces- 

 sary to modify my previous opinions. To this I have already 

 cursorily referred, namely, the depressed interspaces on the 

 valvular surface of Pleurosigma balticum and the rest of the rectan- 

 gularly marked series. But I am by no means prepared to speak 

 positively regarding this or the difference presented between the 

 outer and inner surfaces of the rectangularly or quincuncially 

 arranged group. The existence of a difference is extremely pro- 

 bable for several reasons. Thus the direction of the lines of frac- 

 ture characteristic of each group is hardly reconcileable with any 

 other hypothesis than that the lines constitute the thinnest and 

 weakest portions of the structure ; and assuming this as a gene- 

 rally admitted fact, whether the interspaces are occupied by 

 elevations or depressions, it follows that these cannot occur 

 equally on both sides, otherwise the entire substance would be 

 moniliform, which has never been asserted, to my knowledge. 



It might naturally be supposed that elevated portions occur- 

 ring on one surface of a valve would impart the appearance of 

 depressions when viewed from the opposite surface. But I have 

 failed to satisfy myself of any such distinction. In this dilemma 

 I caused to be prepared two models in plate glass, on one surface 

 of which were carefully imitated the markings of the two typical 

 forms, the other surface remaining plain. These were examined 



* Journal of Microscopical Science for April 1863, plate 8. fig. 11, 



24* 



