274 THE AMERICAN MONTHLY [Oct 
objectives, and it was found that all the species repre- 
sénted, with the exception of the two Auwlacodiscti, be- 
came fainter as the refractive index was increased up to 
about 1.435, when they were entirely invisible, except 
where in contact with the cover-glass. As the index of 
the medium surrounding them was increased above this 
point they became more distinct, the coarser forms being 
almost opaque in oil of cassia, This is exactly what 
should be expected, either on theoretical grounds or based 
on previously published experiments, but in the case of 
the two species of Aulacodiscus mentioned, the distinct- 
ness of visibility under a low power seemed to increase 
from the start, and in the medium where other forms 
disappeared they were even more strongly outlined than 
in alcohol, while under an oil immersion-objective no dif- 
ference could be noted in the sharpness and contrast with 
which the secondary structure was shown in any of the 
various fluids, although portions of the internal plates, 
which extended beyond the external plate in broken forms, 
were extinguished with the rest of the diatoms on the 
slide, showing that the anomalous behavior of these spe- 
cies was confined to the external plate, containing the 
secondary structure. Neither heating to redness on 
plantinum foil nor boiling in strong acids has the least 
effect on the appearance of the secondary structure, nor 
is there anything to indicate that its appearance is due to 
difference in composition rather than of structure. With 
the facts at present available it would be useless to haz- 
ard a conjecture as to the true nature of this structure, 
but it may be safely affirmed that in the external plate of 
this group of species of Awlacodiscus we have a structure 
essentially different from that found among other diatoms. 
Aulacodiscus Oreganus is one of the few diatoms that 
show bright colors with central transmitted light. The 
two valves of this species included on slide under obser- 
vation, when examined with a three-fourths-inch objec- 
a ee 
