N. E. BROWN ON THE STRUCTURE OF DIATOMS. 319 



as distinctly separate dots. It is only on this particular species, 

 the name of which I do not know, that I have been able to discern 

 these pores. Upon the far larger P. major and P. nobilis I can- 

 not see any trace of them, although I do not doubt that they 

 exist in these species also, but are probably smaller than in the 

 species in which I discovered them. As seen by myself and 

 friends at a magnification of 3, COO diameters they are as repre- 

 sented at PI. 23, fig. 13. 



Upon the sides or girdle of all species of Pinnularia are to be 

 seen two slender lines, which under sufficient magnification are 

 seen to be composed of a multitude of short transverse lines ; in 

 P. major these average about 60,000 to the inch. These lines I 

 have failed to resolve into distinct dots, although Mr. E. M. 

 Nelson (Journ. Q.M.C., Ser. 2, Vol. VI. p. 144) states that he 

 has done so, and I do not doubt his statement. But at the same 

 time I very much doubt if the clots of which these transverse 

 lines are composed are real pores. The lines are so easily seen 

 that they evidently are much too coarse for pore structure, and 

 my interpretation of the structure of these two lines on the 

 girdle of Pinnularia is, that each line consists of a multitude 

 of very minute cavities placed side by side, similar to those seen 

 in the front view of the valve, and that when they are truly 

 resolved each cavity will be found to have a minute pore at the 

 centre or a row of pores along the central line of each cavity or 

 clear space between every pair of short transverse lines. 



It may be well to state that there are sometimes appearances 

 to be seen on the walls of the cavities of P. major and P. ?iobilis 

 which may easily be mistaken for rows of pores. As I have 

 seen them, they appear like two rows along each cavity, but 

 upon moving the mirror slightly these rows move also, and 

 clearly demonstrate that they are only diffraction images. 

 The true pores of these species, when discovered, will, I believe, 

 be in one central row. 



Pleurosigma balticuni. In a paper recently published in the 

 Journ. Q.M. 6'., Ser. 2, Vol. XII. p. 155, Mr. T. O'Donohoe has given 

 an account, accompanied by some excellent photographs, of certain 

 details of structure of this diatom as seen in a strewn slide 

 mounted in realgar belonging to Mr. B. J. Capell. By the 

 courtesy and kindness of Mr. Capell I have also had the privilege 

 of examining this slide, and am fortunate enough to be able to 



