232 MEMORANDA. 



The ITIarkings of the Pleurosigma, &c. In the ' Memoranda' 



of the last number of your interesting Journal, a short notice 

 was inserted on the " curious effect of moisture on the mark- 

 ings of the Pleurosigma^^ in which I spoke of the subject of 

 the markings as one, " still, I presume, involved in obscurity." 

 When this was written, I had not seen the following quotation 

 from the ' Philosophical Magazine' for January, 1855, which 

 occurs in an abstract of a paper read before the Royal Society, 

 "On the Structure of certain Microscopic Test-objects, and 

 their Action on the Transmitted Rays of Light." By Charles 

 Brooke, M.A., F.R.S. The passage is as follows : " The 

 * dots' have by some been supposed to be depressions ; this 

 however is clearly not the case, as fracture is invariably ob- 

 served to take place between the rows of dots, and not through 

 them, as would naturally occur if the dots were depressions, 

 and consequently the substance thinner there than elsewhere." 

 Now when writing the former notice, and quoting a passage 

 from the introduction to the ' Micrographic Dictionary' now 

 publishing, in which a diametrically-opposite view is main- 

 tained, I was not prepared to find such eminent observers as 

 t!ie authors of the above-mentioned work, and Mr, Brooke, 

 taking such loholly irreconcilable views, and each, moreover, 

 basing his view upon wholly irreconcilable statements of the 

 same fact, namely, the course of the line of fracture of the 

 shell. The observation of this fact is, I should think, simple 

 and easy to an experienced observer who possesses our most 

 j)Owerful objectives, and I am not without hope that the juxta- 

 position of two such contradictory statements made by such 

 experienced observers, will induce some one qualified by ex- 

 perience, and possessed of sufficient instrumental means, to 

 bring forward such undeniable evidence of the truth of either 

 one statement or tlie other, as may set at rest the question of 

 the course of the line of fracture ; which will be a step, and an 

 important step, towards ascertaining the precise nature of the 

 markings. I may add, that in those species of the genus 

 Pleurosigma, which have the " stria? transverse and longitu- 

 dinal," such as the P. Hippocampus, I have observed that 

 when moisture has insinuated itself between the glasses in 

 which they are mounted, the boundary line between di'yness 

 and moisture has always a tendency to arrange itself in lines, 

 either transversely across or along the shell, which in this case 

 are manifestly parallel to the two directions of least distance 

 of the dots. Granting that the Diatomaceae belong to the 

 Vegetable Kingdom, the nature of these dots, may, I think, 

 be added to the list of questiones vexatec in vegetable anatomy 

 adduced by Mr. F. Currey, in his paper in the ninth number 

 of this Journal, " On the Spiral Threads of the Genus Trichia," 



