them often before, "witboiit being aware of their nature. Like the 

 corresponding plate of Heliopelta, this is composed of spherical gran- 

 ules of silex, but instead of being in close contact, they are distant, 

 and joined or cemented together by a thin plate of silex, the arrange- 

 ment and place of the particles being governed by that of the hexa- 

 gons of the outer plate, one granule being placed against each hexa- 

 gon. By careful adjustment of the focus of the instrument, with a 

 power proportioned to the size of the areolae, the granules can be 

 seen in the centre of the hexagons ; care must, however, be taken 

 not to confound an optical effect with the appearance of the granules ; 

 each areola is a minute lens, and so refracts the light as to give a 

 brio-ht or dark dot as the focus is changed, and the granules them- 

 selves contribute to this effect. Practice, however, will enable one to 

 distinguish these effects. 



The species Eupodiscus Argus and Rogersi present strong evi- 

 dence of the inner plates ; so, also, do some specimens of Isthmia ner- 

 vosa, of Ejnthemia, Achnanihes, and Polymyxus coronalis. I think I 

 have seen indications of them in several other genera. In some of 

 the Pinnularia and Navicula there are appearances which I can ex- 

 plain only on the supposition that the valve is composed of two plates, 

 as suggested by Schleiden. Sufficient, I think, has been proved to 

 warrant the generalization that the valve of the Diatomacese consists 

 of at least two plates of silex, the inner one of a structure more or 

 less differing from that of the outer, giving that peculiar appearance 

 to those species described as veiled, — partly the cause of the dots in 

 the hexagonal areolae of some species, — and often, probably, explain- 

 ing the varying descriptions and figures of different writers. 



There is a difference of opinion among Diatomists as to the shape 

 of the dots or marks of the very finely marked kinds, such as the 

 whole of the genus Pleurosigma (Smith), — Gyrosigma (Hassal), 

 Mr. Wenham, by magnifying photographs of P. angulatum to 15000 

 diameters, has proved, as I think, that the areolae of that species 

 (and undoubtedly of all the species with diagonal lines) have hex- 

 agonal areolae, exactly like those of Coscinodiscus. Professor O. N. 

 Rood, of Troy, by the same process, has obtained photographs of the 

 same species (7000 diameters), which he thinks prove the areolae 

 to be circular. Professor Rood's photographs show some indications 

 of the hexagonal form, and I believe the difference between his 

 figures and Mr. Wenham's must be owing to some difference in the 

 manipulation. The areolte of the coarsely-marked forms being un- 

 questionably hexagons, it is probable, from analogy, that those of the 

 finer forms are so also. Mr. Wenham, as quoted by Professor Rood, 

 " states that he has ascertained by a -^oth that the markings of this 

 object are due to spherical particles of quartz." (Am. Jour. Science, 



