698 Transactions of the Society. 



XVII. — On the Resolution of New Detail in a Coscinodiscus 



aster omphal us. 



By Edwaed M. Nelson. 



{Read October 19, 1910.) 



Additional work with the new lens, mentioned in my last note.* 

 lias pushed diatom resolution on a step further. Not only have 

 known images of a very difficult character been more easily seen, 

 but detail never previously glimpsed has been discovered. This 

 new detail is a fine sieve covering the eye-spot of a Coscino- 

 discus aster omphalus. It will probably be considered sufficiently 

 important to be recorded, because it may be said to complete this 

 diatom, and constitute it the first diatom of its class whose form is 

 fully known, so that an enlarged model of it might be made. 



The simplest forms of diatomic structures seem to be those of 

 the Isthmia, Biddulphia class, which have single walls with areo- 

 lations covered by a sieve membrane. A model of an Isthmia 

 might be made out of a leather cigar-case by making holes in it 

 with a gun-wad punch and covering these holes with gauze ; a 

 protuberance open at the top, also covered with gauze, must be 

 made at opposite corners. The Coscinodiscus aster omphalus, how- 

 ever, consists of a polygonal cell-like structure (fig. 105, A) not 

 unlike honey-comb, the top of these cells being covered with cir- 

 cular or oval-shaped caps (B).| 



These caps have at their peripheries a circlet of large circular 

 perforations, each of which is closed by a very fine sieve membrane.^ 

 In the space inside this circlet is another sieve membrane with a 

 larger mesh. At the bottom of the cells or honey-comb is a plate 

 having a hole, known as the "eye-spot," in the centre of each 

 cell (A and C). 



The true form of this hole, or rather short pipe, was described 

 by Mr. H. Morland at the Quekett Club.§ It is on the lower end 

 of this pipe that the new sieve membrane has been discovered (C). 



* Page 147 ante. 



f Fig. B is diagrammatic, the spots in the peripheral circles are small, but not 

 quite as small as those in C. The next inner circle of black dots should be repre- 

 sented about the same size as those at present in the peripheral circles and like 

 them of irregular shape. They are the exterior holes of the central sieve pattern. 



% Journ. Quekett Micr. Club, vii. ser. 2 (1898) p. 81, pi. viii. fig. 8. This very 

 fine sieve is the membrane described in rny previous note as being seen for the first 

 time in a balsam mount. 



§ Op. cit., iii. ser. 2, p. 79, and for illustrations see Trans. Middlesex Nat. Hist. 

 Soc., 1889, photo plate, fig. 2 (see No. 46 in list). 



