322 PROCEEDINGS OF THE ACADEMY OF [May, 



ination of fragments in which the plates were separated indicated, 

 however, that the typical "honeycomb" cellular structure was 

 likewise present in these species, but masked by the unusual char- 

 acter of the external plate, which differs from that of other dia- 

 toms in having the finer secondary structure between, rather than 

 over, the large cells of the middle plate. 



Recently, with the view of further determining the relations of 

 this structure to that of other species, a special mount was pre- 

 pared, including A. Oreganus, A. RogersH, with typical species of 

 Coscinodiscus, Triceratlwn, Actinocydus, Actinoptyclius, 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 fluids of varying refractive indices to be passed 

 under the cover and withdrawn by the use of blotting paper in the 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 objectives, and it was found that 

 all the species represented, with the exception of the two Aulaco- 

 discii, became fainter as the refractive index was increased up to 

 about 1.435, when they were entirely invisible, except where in 

 contact Avith the cover glass. As the index of the medium sur- 

 rounding 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 have been expected, either on theo- 

 retical grounds or based on previously published experiments, but 

 in the case of the two species of Aulacodiscus mentioned the 

 distinctness 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 difference 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 j^lates, which 

 extended beyond the external plate in broken forms, were extin- 

 guished with the rest of the diatoms on the slide, showing that the 

 ■anomalous behavior of these siiecies was confined to the external 

 plate, containing the secondary structure. Neither heating to 

 redness on platinum 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 diflerence in 

 composition rather than of structure. With the facts at present 

 available it would be useless to hazard a conjecture as to the true 

 nature of this structui-e, but it may be safely affirmed that in the 

 external plate of this group of species of Aulacodiscus we have a 

 structure essentially difierent from that found among other diatoms. 



