PROCEEDINGS OF SOCIETIES. 217 
that peculiar appearance to those species described as veiled,—partly 
the cause of the dots in the hexagonal areolz of some species,—and 
often, probably, explaining 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 15,000 
diameters, has proved, as I think, that the areole of that species 
(and undoubtedly of all the species with diagonal lines) have hexa- 
gonal areole, 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 
areolee to be circular. Professor Rood’s photographs show some 
indications of the hexagonal form, and I believe the difference be- 
tween his figures and Mr. Wenham’s must be owing to some 
difference in the manipulation. ‘The areolee of the coarsely marked 
forms being unquestionably 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 =4th that 
the markings of this object are due to spherical particles of quartz.” 
(‘Am. Jour. Science,’ Nov., 1861, p. 336.) This observation, with 
the discovery of the inner plate of the Coscinodiscus, and its struc- 
ture, makes the analogy of the structure of the two genera complete, 
and may be considered as proving the existence of the inner plate 
in this genus. 
Another point in the structure of the valve has been a subject of 
much difference of opinion—some contend that the areole are ele- 
vations, others that they are depressions. Dr. J. W. Griffiths gives, 
in the ‘ Micrographical Dictionary,’ his reasons for considering them 
to be depressions. I have reasons for thinking that neither party 
has the true explanation of the structure. My opinion is that the 
exterior of the shell is smooth or nearly so, and that the borders of 
the hexagons, or other shaped areolee, and costze of the costate forms, 
are internal projections from the outer plate, as on the under side 
of the leaf of the Victoria Regia, intended to give strength to the 
cell with the smallest quantity of material. This will explain the 
trace of the hexagons seen on the inner plate of Heliopelta, as only 
the projecting wall of the areole would come in contact with the 
inner plate. Dr. Griffiths reasoned that the areolee were depressions 
because they were the thinnest parts of the shell; the faets are 
correct, but the inference may not be, as there is another explanation 
of the phenomena. 
In company with Dr. C. T. Jackson, I have dissolved a shell of 
Coscinodiscus under the microscope, with caustic potash, and found 
that the area of the cellules was dissolved before the walls, and that 
therefore they are the thinnest parts, as Dr, Griffiths judged from 
the optical effect. 
