1901] MICROSCOPICAL JOURNAL. 273 
Structure of Diatoms. 
FRANK J. KEELEY. 
In studying the structure of diatom valves some years 
ago, the method employed, mounting broken valves at 
right angles to the cover glass, proved efficient for most 
of the coarsely marked forms, but failed with certain 
species of Aulacodiscus. | 
Such forms as A. sollittianus, A. margarataceous, etc., 
yielded satisfactory sectional views and proved not to 
differ materially in structure from Coscinodiscus but an- 
other group, including A. oreganus, A. rogersti, A. jani- 
schii, etc., proved too opaque for the elucidation of their 
structure by this method. Further examination of frag- 
ments in which the plates were separated indicated, how-. 
ever, that the typical “honeycomb” cellular structure was 
likewise present in these species, but masked by the un- 
usual character of the external plate, which differs from 
that of other diatoms in having the finer secondary struc- 
ture between, rather than over, the large cells of the mid- 
dle plate. 
Recently, with the view of further determining the 
relations of this structure to that of other species, a spe- 
cial mount was prepared, including A. oreganus, A. ro- 
gersii, with typical species of Concinodiscus, Triceratium, 
Actinocyclus, Actinoptychus, etc. The various forms were 
arranged in a line on a square cover-glass, supported on 
the slide by bands of cement at two opposite edges, thus 
‘permitting the fluids of varying refractive indices to be 
passed under the cover and withdrawn by the use of blot- 
ting paper in the manner familiarly known as‘‘irrigation.” 
The fluids employed consisted of absolute alcohol, cedar 
oil, oil of cassia and mixtures of same, giving refractive 
indices from about 1.37 to over 1.60. Starting with the 
lowest refractive index, the appearance-of each diatom 
was carefully noted under low, medium and high aperture 
