430 E. M. NELSON ON MINUTE DIATOM STRUCTURE. 



The length of the stalk measures '00124 inch from its base to the 

 top of the rose of the watering-pot, the length of the valve being 

 •0079, and its breadth -0048. The ridges radiating from the 

 centre of the valve between the rows of large areolations are 

 caused by a thickening of the silex, the areolations being in a 

 thinner part of the silex. Thus, if any one wished to make a 

 model in wood of this portion of the valve, they ought to take a 

 board of uniform thickness and plough grooves in it ; the holes 

 representing the large areolations would be drilled in these 

 ploughed grooves where the board was thinner. 



On the thick ridges between the rows of areolations there are 

 intercostal dots, but these are very irregular, and numbers of 

 them are missing. So much for the coarse structure. One of 

 the peculiarities of this diatom is the apparent absence of any 

 finely perforated membrane, except on the conical side and convex 

 top of the rose of the watering-pot. These diatoms have been 

 repeatedly searched for some delicately perforated membrane, but 

 hitherto without success. Thanks, however, to the new Powell 

 condenser, we are, although not able to see it, morally certain of 

 the presence of a delicate perforated membrane ; for little 

 projections sticking out from the edges of the large areolations 

 can, with attention, be made out. 



Similar little projections may be seen in the areolations of 

 Coscinodiscus asteromphcdus, where the delicate membrane has 

 been blown out ; in this instance, however, the little projections 

 are much more numerous. Mr. Karop's drawing of similar 

 structures (Figs. 4 and 6, PI. IV., Vol. 3, Ser. II.), and my 

 photographs (Figs. 4, 5, and 6, PI. XVIII., Vol. 3, Ser. II.), will 

 illustrate my meaning admirably. The image is not easy. The 

 interesting point is, that if the coarse part of the secondary 

 structure is such a difficult object, what must the full resolution 

 of the delicate membrane be ? The answer to this question must 

 be left to the microscopy of the future. 



It is, however, satisfactory to be able to report that the 

 question has been more speedily answered than was anticipated ; 

 for this afternoon Dr. Tatham, who was examining this specimen, 

 said, " I can glimpse the perforated membrane." After a little 

 while I was able to glimpse it also. It was, of course, like all 

 similar new resolutions, merely a glimpse object, quickly lost 

 again. But after modifying the adjustments and the illuminating 



