282 SURFACE-MARKINGS OF DIATOMACEJE. 



terpretation of the reversal of the lights and shades produced by- 

 alteration of focus, harmonizes also with the varieties of aspect pro- 

 duced by different modes of illumination (Fig. 120, A, b, c, d). And 

 it derives additional confirmation from several incidental circum- 

 stances ; such as from the fact that the lines of fracture, instead 

 of traversing the areola?, here follow the course of the intervening 

 network ; and that when specimens mounted beneath glass have 

 had their markings obscured by moisture, the obscurity is dis- 

 sipated by the application of a gentle heat, in a way that is readily 

 explicable on the supposition that the markings are elevations, but 

 is wholly unintelligible on the idea of their being depressions.* 

 Moreover, that these minute markings are not to be interpreted by 

 the analogy of the coarser network, is made obvious by the fact 

 that they frequently co-exist in the same shells ; thus, in certain 

 species of Triceratium, Coscinodiscus, and Actinocyclus, the floors 

 of the hexagonal depressions are studded with markings resembling 

 those of a Plearosigma ; and these are particularly conspicuous in 

 the beautiful Heliopelta (Plate I., fig. 3). There is reason to be- 

 lieve, indeed, that in these and other instances the two sets of 

 markings belong to two distinct layers ; the coarser areolation 

 belonging to the external, whilst the fine granulation, which gives 

 rise to the appearance that has caused Ehrenberg to describe several 

 Diatoms as ' veiled,' belongs to the inner. + 



217. The process of Multiplication by binary subdivision takes 

 place among the Diatomacea? on the same general plan as in the 

 Desmidiaceoe, but with some modifications incident to the peculi- 

 arities of the structure of the former group. — The first stage con- 

 Paper was read before the Microscopical Society) as the result of observa- 

 tions made with an Objective of l-50th inch focus and large aperture, 

 that the valves are composed wholly of spherical particles of silex, pos- 

 sessing high refractive power ; and he showed how all the various optical 

 appearances presented by the different species could be reconciled with 

 the supposition that their structure is universally the same. Mr. W. has 

 succeeded in obtaining distinct impressions of the surface-markings by 

 the Galvano-plastic process (See " Quart. Journ. of Microsc. Science," 

 Vol. iii., 1855, p. 244).— The opinion of Prof. Max Schultze, however, by 

 whom this subject has been very elaborately investigated, does not har- 

 monize with the foregoing. He affirms that "neither spherical, conical, 

 nor pyramidal elevations are the cause of the punctated appearance, 

 although the decussating sets of ridges may at the points of intersec- 

 tion afford an appearance resembling that of tubercular elevations." And 

 he considers that the sculpturing, both in the coarsely and in the finely 

 marked Diatom- valves, though at first sight allied to what is seen on the 

 surface of artificial siliceous pellicles, is in reality due to wholly diffe- 

 rent conditions. (See his Memoir " Die Structur der Diatonieenschale," 

 and the Abstract of it in "Quart. Journ. of Microsc. Science," Vol. iii., 

 N. S., 1863, p. 120.) 



* See Mr. G. Hunt in "Quart. Journ. of Microsc. Sci." Vol. iii. (1855), 

 p. 174. 



t See Mr. C. Stodder (of Boston, U. S.), "On the Structure of the 

 Valve of the Diatomacece," in " Quart. Journ. of Microsc. Science," 

 VoL iii., N. S. (1863), p. 214; also Ralfs, Op. cit., Vol. vi. (1858), p. 214; 

 and Rylands, Op. cit., Vol. viii. (1860), p. 27. 



