DIATOMACEJ2 



The nature of the delicate markings with which almost every 

 diatom frustule is beset has been one of the most interesting in- 

 quiries of the students of these forms since the introduction of the 

 homogeneous, and especially the apochromatic, objectives; and it 

 cannot be doubted that certain peculiarities of structure have been 

 demonstrated which were never before seen. In the present state 

 of the theory and practice of microscopy it would be extremely 

 unwise to give absolute adhesion to any present interpretation of 

 what is now held by some students of diatom structure of no mean 

 repute and of unrivalled manipulative skill to be the absolute struc- 

 ture of some of the larger forms!*' 



Thus, concerning the group Coscinodiscece, representing the most 

 beautiful of the discoid forms of the whole group of Diatomacece, we 

 represent in Plate I, fig. 3, a photo-micrographic image of Costino- 

 discus aster omphal us magnified 110 diameters. But in fig. 441 

 the areolce of this diatom are 

 seen under great magnification 

 with recent powers. It is 

 contended that the diatom, 

 although consisting of a single 

 siliceous membrane, has a 

 double structure, viz. coarse 

 and fine areolations, the latter 

 within the former ; and there 

 appears little reason to doubt 

 this. The coarse areolations 

 are for the most part circular 

 in outline, and the intervening 

 silex is thick. Inside these 

 areolations is an extremely 

 delicate perforated membrane, 

 the outer row of whose perfora- 

 tions are larger than the rest. 

 From the very delicacy of this 

 membrane, and its consequent 

 easy fracture, it is often want- 

 ing. In Plate I, fig. 4, we present a photo-micrograph of the same 

 object magnified 2,000 diameters. 



In Isthmia nervosa, a side and front view of which are seen in 

 fig. 457, a similar construction is discoverable. In this diatom the 

 coarse areolations are very large and the silex correspondingly thick ; 

 but the inner membrane is excessively thin and delicate. The per- 

 forations are large and irregular in shape around the margin, but 

 small and circular in the centre. In fig. 443 the form of areola- 

 tions is shown, and a broken membrane seen, with the fracture 

 passing through the perforations. 1 



Xot less interesting is the beautiful form Aidcicodiscus Kltton'd; 

 a photo-micrograph of this magnified 270 diameters is seen in Plate 

 I, fig. 5 ; while a small portion of the centre of a kindred form, 



1 Note on the finer structure of certain diatoms, E. M. Nelson and G. C. Karop, 

 Junrn. Quekett Club, vol. ii. ser. ii. p. 269. 



FIG. 441. -Magnification, of ' ultimate struc- 

 ture ' of Coscinodiscus aster omphalus, 

 from a drawing by Messrs. Nelson and 

 Karop (Jour)i. Quekett Club, vol. ii. 

 ser. ii. p. 269). 



