DIATOMACE.E 



591 



markings with which almost every 



interesting 



111- 



441 



diatom, 

 a 

 has 



single 



a 



The nature of the delicate 



diatom frustule is beset has been one of the most 

 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 110 mean 

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

 ture of some of the larger forms. 



Thus, concerning the group Coscinodiecece, representing the most 

 beautiful of the discoid forms of the whole group of Difit<>i,i<in--<i', we 

 represent in Plate I, fig. M, a photo-micrographic image of I'osti- 

 il^cn-s astei-oui/t/nil us magnified 110 diameters. But in lig. 

 the areolcn of this diatom are 

 .-eBn under great magnification 

 with recent powers. It is 

 (.intended that the 

 although consisting of 

 siliceous membrane, 

 double structure, vix. 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 

 object magnified 2,000 diameters. 



In Isthmia nen-osa, 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 



Not less interesting is the beautiful form Aulacodiscus Klttonii; 

 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. <_'. Karop, 

 Joitrn. Qxekett Club, vol. ii. ser. ii. p. tit'i'.t. 



Fi. 441. -Magnification of ' ultimate struc- 

 ture ' of Cuscinodisciin aster omphalus, 

 from a drawing by Messrs. Nelson and 

 Karop (Juiirn. Qitekftt Club, vol. ii. 

 ser. ii. p. 2(59). 



photo-micrograph of the same 



