1884-85-] EdiiiburglL Naturalists Field Club. 241 



as to enable it to return to its original condition when bent or 

 rolled up. With regard to the character of the siliceous lamina, 

 Nilgeli has spoken of it as extra-cellular, while Meueghini has 

 maintained that the silica must permeate some form of connective 

 or organic membrane. Smith, ^ on the other hand, has held that 

 by macerating the frustules in acid, the cell membrane partly or 

 wholly becomes detached from the valves, and has thereby de- 

 duced that the siliceous coat is an independent structure. 



The sculpturing of tlie valves is often of the most elegant and 

 beautiful . description. It may appear in the form of distinct ribs 

 or costaj, of more or less minute and well-defined dots or puncta, 

 of stripes or stria? of very variable arrangement, or of " cells " or 

 areolte. More than one kind of marking may occur on one and 

 the same valve ; and the ornamentation may cover the entire 

 frustule or may leave certain areas unaffected. The real nature 

 of the various types of sculpturing thus indicated has been very 

 variously stated. A point has sometimes been ascribed to a 

 depression, sometimes to an elevation, and sometimes to a conden- 

 sation or thickening of the siliceous matter. The costaB have in 

 some instances been attributed to furrows, in others to ridges, and 

 in yet others to canals or to incomplete internal partitions or 

 septa. The striae are in some cases resolvable into dots or 

 puncta, and are called monll'tform ; but in others they cannot be so 

 resolved, and are said to be continuous. Recently Dr Flogel,- from 

 a series of microscopic sections of frustules which he has been 

 able to procure, stated the following general conclusions : — 

 The marking is caused — ■ 

 (I.) By sharply projecting wall thicknesses — 



(a) on the inner surface of the membrane, e.g., Achnanthes 



(transverse stria?), Isthmia (valves) ; 

 (/?) on the outer surface of the membrane, e.g.., Isthmia 

 (girdle band). 

 (II.) By chambers within the membrane — - 

 (a) with distinct openings — 



(i) on the outer surface of the cell, and closed in- 

 wards, e.g., Tiiceratium, Coscinodiscus ; 

 (ii) on inner surface, chambers large, e.g., Pinnularia. 

 (/3) Without distinct openings, but of considerable size— 

 (i) with smooth chamber-walls, e.g., Coscinodiscus 



centralis ; 

 (ii) with nodular thickened chamber - walls, e.g., 

 Eupodiscus. 

 (y) Closed on all sides and extremely small, approaching 

 the limit of discrimination, e.g., Pleurosigma, Navi- 

 cula lyra, Surirella. 

 1 ' Ann. Xat. Hist.,' 1851. 2 < j^ur. Roy. Micr. Soc.,' 1884. 



