81 



On Diatom Structure. 



By Edward M. Nelson, P.RM.S. 



(Read May 20th, 1898.) 



Plate 8. 



As time progresses fresh discoveries in diatom structures become 

 more difficult, for the simple reason that the larger or coarser 

 structures have all been described long ago, and only the finer 

 and more delicate ones are left. Among these, however, there is 

 a good deal yet to be done, even with our present appliances, and 

 when further optical improvements are effected there will be 

 a great deal more to do. For of this we may be certain, that 

 microscopes have by no means exhausted the details of diatom 

 structure, any more than telescopes have exhausted the details of 

 the solar system. 



The first object to be recorded this evening is a variation 

 of Coscinodiscus Asteromphcdus (Fig. 1, PI. 8). Instead of the 

 well-known pattern consisting of a ring of larger areolations sur- 

 rounding the finely perforated membrane which covers a large 

 polygonal cell, we have a circular ring of brackets projecting 

 inwards to strengthen this delicate membrane or cover. The 

 perforations in the centre of this membrane are excessively 

 minute, and those round it are far more minute than those in 

 an ordinary Asteromphcdus. These brackets are very similar in 

 appearance to those supporting the delicate auditory membrane 

 in the pygidium of a flea. 



The second point to record is the discovery of a tertiary 

 structure in an Asteromphcdus of the common form (Fig. 8, PI. 8). 



This tertiary structure must be looked for in the interior of 

 those secondaries which encircle the finely perforated membrane. 

 It is hardly necessary to say that these tertiaiies are excessively 

 minute. In form they are not unlike the secondaries of Astero- 

 lampra vulgaris, admirably figured by Mr. Karop in Fig. 10, PI. -t, 

 Ser. 2, Vol. 3, p. 41 (1887). 



JouR^^ Q. M. C, Series II., No. 43. 6 



