60 BRITISH OHAEOPHYTA. 



they are crowded into disorder, penetrate deeper into, 

 the tissue, and in some species become thick and 

 spongy in appearance (Plate V, figs. 8, 9). 



The reticulate series also exhibit a progressive- 

 development. The lines which mark the reticulation 

 begin by lying flat on the surface, but as they grow 

 they develop little protuberances which in the case of 

 N. tenuissiina are very conspicuous (Plate V, figs. 3, 4). 



The nature of the decorative substance calls for 

 further investigation. The examination of the tuber- 

 cles, reticulations, and granules in profile seems to 

 suggest that in all three cases they are formed of a 

 colourless gelatinous substance resting on a smooth 

 pigmented base. This view is supported by the- 

 detection now and again of a piece of colourless- 

 membrane bearing the characteristic decoration and 

 having the appearance of a fragment detached from 

 its base. 



Immediately below the outer coloured 

 nner membrane lies a second so closely 



membrane adherent as often to requii'e the aid of 

 a reagent such as chromic acid to- 

 separate them (Plate IV, f. 9; PI. V, f. 5). This, 

 membrane differs but little in colour in the different 

 species, varying from very lightly-tinged yellow in 

 the more translucent membranes to light yellow- 

 brown in those which are more opaque. In texture- 

 it is very thin and flexible, frequently lying flat when 

 separated and placed on the microscope slide. Not 

 infrequently it exhibits traces of the ridges of the 

 overlying membrane.. The decoration of this sub- 

 membrane is uniformly granulated, the granules being 

 always faintly, usually very faintly, indicated. The- 

 granular character apparently exists even in species. 



