722 Transactions of the Society. 



XXIV. — The Structural Detail of Coscinodiscus aster omphalus. 

 By T. W. Butcher, M.B. CM. F.ll.M.S. 



(Read October 18, 1911.) 

 Plates XXV. to XXVII. 



The note read by Mr. Nelson at the Meeting cf the Boyal Micro- 

 scopical Society on October 19, 1910,* announced the completion of 

 over twenty years' labour on this diatom, and contained the sug- 

 gestion that an enlarged model of it could now be made. I take 

 this opportunity, therefore, of submitting photomicrographs of 

 various specimens in my possession. 



Series I. — This displays views in series of a single field, taken 

 at different consecutive foci of the various well-known features — 

 the grouping of the primary areolations, the coarse secondaries 

 (with their finer tertiaries), and the delicate siliceous meshwork 

 woven like a net over the outer surface of the valve in the mature 

 state. 



Series II. — Under Series II. are classed fractures — either a 

 single one of special interest, where the outer perforated cap is dis- 

 lodged from the rest of the valve almost in its entirety, with its 

 structure obvious, as it comes into focus in an otherwise clear space ; 

 or a fracture-margin displaying its edges in varied form ; or a field 

 where one portion is in focus, whilst other portions, lying somewhat 

 lower down in the mount, give one the appearance of the " eye-spot 

 layer" out of focus. 



Series III. A and B. — This series demonstrates the tertiaries 

 described by Mr. Nelson,f and, without re-opening the point as to 

 whether they could be " ghosts " or phenomena of interference, it 

 is self-evident that their irregular number and position in the 

 secondary markings destroys any doubt as to their structural 

 reality, and there is no necessity to enter into any elaborate exercise 

 in geometrical design with Mr. Nelson in elucidating the pattern on 



* See this Journal, 1910, p. 698. 



f Joum. Quekett Micr. Club, Series 2, vii. p. 81. 



EXPLANATION OP PLATE XXV. 



Fig. 1. — Primary areolations ; the central perforated membrane surrounded by 

 a ring of coarse " secondaries," within which are the fine tertiaries. 

 X 2700. 



,, 2.— Fracture displaying a single perforated cap. x 1000. 



„ 3. — Fracture displaying group of perforated caps lying on the upper surface 

 of another larger fragment. At three consecutive foci, x 1000. 



„ 4. Ditto. Ditto. 



„ 5. Ditto. Ditto. 



