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On the Minute Structure of some Diatom ace.e from 

 CoRicA Bay, Melbourne. 



By a. a. Merlin, F.R.INLS. 

 (^Read January Idth, 1900.) 



Plate 17. 



A " circle " slide of selected Diatomacese from Corica Bay, 

 although containing only well-known forms, has yielded some 

 results under the highest powers which have interested me, and 

 may prove new to other members of the Club, even should the 

 points referred to have been previously observed. I feel that in 

 any case I can rely upon their indulgence in broaching the old 

 but ever-fascinating subject of diatomic structure, regarding 

 which all advances hitherto made in the perfection of our 

 objectives, and in the refinements of critical illumination, have 

 served but to open vistas of new detail, and have left us apparently 

 still far from any demonstrable solution of the mechanical cause 

 of the life motions of these organisms. 



The |-inch Zeiss apochromat of N.A. 1*4, illuminated by the 

 full axial cone of Powell's dry apochromatic condenser, was 

 employed in all the following observations, the diatoms being in 

 a medium of somewhat high refractive index, probably mono- 

 bromide of naphthaline and balsam. 



The preparation contains several specimens of Bidduljihia 

 (B. reticukda Ptoper ?) exhibiting complicated structure. Each 

 areolation of the valve appears to be surmounted by an outer 

 domed silicious film of great tenuity, on which extremely fine 

 secondaries of the Coscinodiscus asteromphcdus type have been 

 glimpsed (Fig. 1), A fracture extending across one of these caps 

 has admirably served to demonstrate both its delicacy and reality. 

 On focussing down just sufficiently to render the image of the 

 outer cap invisible, a second silicious plate comes into view (Fig. 2) 

 decidedly more substantially built than the first, pierced by 

 distinct, well separated, and fairly equidistant perforations, of 



