24 On the Strudure of Diatoms. 



complicated structure, as yet beyond the reach of the instrument. 

 The extreme fineness of the longitudinal lines, as compared with 

 the transverse, reminds one of the Navicula cuspidata, and as 

 is the case with the coarser shell, no efibrts avail to develop a 

 semblance of beading. 



Stricdella imipunctata. — Two sets of fine lines, and as the 

 direction of the light is changed, may be made to exhibit either 

 beads or squares. In point of value as a test, will be found to 

 approach Surirella gemma. 



Grammaiophora. — Of this genus the writer has examined the 

 G. marina, G. suhtilissima, and G. serpentina; all of which are 

 resolved into hexagons. Broken specimens of G. marina show the 

 line of fracture running through the hexagonal planes and leaving 

 points of the network projecting. The markings continue com- 

 pletely illustrated as the stage is revolved, in whatever direction 

 the beam of light may fall. 



Stauroneis. — Some of the larger varieties of S. phoenicenteron 

 are covered with hexagonal areolae, easily exhibited with central 

 daylight. The projecting points of the fractured partitions between 

 the hexagons may be observed. 



Pleurosigma anguJatum. — Hexagons. The line of fracture 

 generally running around them, but quite often through them. 



Pleurosigma Balticum. — A drop of water slowly advancing by 

 capillary attraction shows this shell to be covered with squares, 

 and proves that both sets of lines forming the boundaries of the 

 squares are on the same surface of the valve ; and the appearance 

 presented by an air bubble on the other side proves that surface 

 to be smooth. 



Pleurosigma formosum. — Near the ends of the frustule it is 

 easy under certain adjustments of the light to make it appear like 

 a checker-board with alternate bright red and green squares. 

 Double rows of green and red beads alternating may be seen on 

 this as well as on other species of the same genus.* When we 

 resort to central light from a white cloud, and thus lessen the 

 liabihties to err caused by refraction, difiraction, decomposition of 

 light, and oblique projection of shadows, the conclusion is arrived 

 at that these various appearances are caused by two sets of inter- 

 secting diagonal ridges, the finer ridges running up and down, over 

 and between the coarser, and subject to considerable variation even 

 on the same frustule. This theory would also account for the 

 " beads " (?) being of different colours, and the same " beads " 

 changing colour when the focus is changed. We see in many of 

 the mollusks shell-markings of a similar character. 



Concluding remarlcs. — It would seem that the perfect box-like 

 form of the shells of the Diatomaceae and their elaborate oma- 

 * Dr. Pigott, in ' M. M. J.' 



