N. E. BROWN ON THE STRUCTURE OF DIATOMS. 327 



of the hexagonal meshes I can sometimes make out a diamond- 

 shaped arrangement of dark dots as seen in fig. 5.* 



In this species, as in P. balticum, wherever a junction of two 

 or more bars of a grating occurs, there a black dot is seen, due to 

 diffraction or refraction of the node so formed. And in my 

 opinion wherever grating structure occurs, the nodes may be 

 expected to appear as black dots. 



At one place a fragment of the valve is broken off and turned 

 edgeways to the cover-glass. This edge-view shows the two 

 gratings distinctly, but at the same time, owing to the shadow of 

 the mass, I am quite unable to see how they are connected to each 

 other. But from an examination of this piece, as well as of the 

 valve where the outer grating is stripped off, it is evident that 

 the faint brown colour peculiar to this diatom resides in the outer 

 grating, the inner one being colourless. 



I cannot, however, confirm Mr. Nelson's statement (Journ. 

 Q. M. C, Ser. 2, Vol. XII. p. 99) that the meshes of the outer 

 and inner grating alternate with one another, for in this particu- 

 lar specimen I think there can be no question that the meshes of 

 the outer grating are exactly superposed over those of the inner 

 grating when seen with exactly central light. I have tested them 

 several times by the unaided eye and by means of a micrometer 

 in the eye-piece, and always found them to correspond, except 

 when the light was not absolutely central. Also the edge- view 

 confirms their superposition so far as I have been able to make it 

 out, but it is very difficult to get a really good focal image of this 

 part. 



P. angulation has one very obvious peculiarity which I do not 

 remember to have seen mentioned, namely, that at the ends of the 

 valve the grating suddenly changes from the hexagonal to the 

 square type of mesh. This should form a good specific character. 



Surirella gemma. Mixed with Phurosigma balticum on Mr. 

 Capell's slide are numerous specimens of Surirella gemma, which, 



* Mr. T. F. Smith is of opinion that the outer grating is different in 

 structure from the inner grating, and views of both gratings are given in 

 The, Microscope and its Revelations, 8th ed. p. 593, pi. 1, figs. 1 and 2. I 

 am not able to confirm this view, for every structural image seen on the 

 outer grating I have also"been able to see on the inner grating it is merely 

 a question of focus and illumination. The "delicate membrane" on the 

 outside of the shell described by Mr. Smith I have already noted under 

 P. balticum, so need not make any further remark upon it. 



