GENERAL CHAEACTEES OF DIATOMACEjE. 277 



with any modifications of the contour of the valves, which may be 

 square, triangular (Fig. 119), heart-shaped (Fig. 130), boat-shaped 

 (Fig. 129, a), or very much elongated (Fig. 120), and may be 

 furnished (though this is rare among the Diatornaceae), "with pro- 

 jecting out-growths (Figs. 135, 136). Hence the shape presented 

 by the frustule differs completely with the aspect under which it 

 is seen. In all instances, the frustule is considered to present its 

 ' front ' view when its suture is turned towards the eye, as in 

 Fig. 129, B, c ; whilst its ' side ' view is seen when the centre of 

 either valve is directly beneath the eye (a). Although the two 

 valves meet along the suture in those newly-formed frustules 

 which have been just produced by binary subdivision (as shown in 

 Fig. 121, A, e), yet as soon as they begin to undergo any increase 

 the valves separate from one another, and the cell-membrane 

 which is thus left exposed immediately becomes consolidated by 

 silex, and thus forms a sort of hoop that intervenes between the 

 valves (as seen at c) ; this hoop becomes broader and broader with 

 the increase of the cell in length; and it sometimes attains a very 

 considerable width (a, b). As growth and self-division are con- 

 tinually going- on when the frustules are in a healthy vigorous 

 condition, it is rare to find a specimen in which the valves are not 

 in some degree separated by the interposition of the hoop. 



215. The impermeability of the Siliceous envelope renders neces- 

 sary some special aperture, through which the surrounding water 

 may come into relation with the contents of the cell. Such aper- 

 tures are found along the whole line of suture in disk -like frustules ; 

 but when the Diatom is of an elongated form, they are found at the 

 extremities of the frustules only. They do not appear to be abso- 

 lute perforations in the envelope, but are merely points at which 

 its siliceous impregnation is wanting ; and these are usually indi- 

 cated by slight depressions of its surface. In some Diatoms, as 

 Surirella (Fig. 129) and Campylodiscus (Fig. 130), these inter- 

 ruptions are connected with what have been thought to be minute 

 canals hollowed out between the siliceous envelope and the mem- 

 brane investing the endochrome ; but it seems more probable that 

 the apparent canals are really internal ribs or projections of the 

 shell. — In many genera the surface of each valve is distinguished 

 by the presence of a longitudinal band on which the usual mark- 

 ings are deficient ; and this is widened into small expansions at 

 the extremities, and sometimes at the centre also, as we see in 

 Pleurosigma 'Fig. 120) and Gomphonema (Fig. 140). This band 

 seems to be merely a portion in which the siliceous envelope is 

 thicker than it is elsewhere, forming a sort of rib that seems de- 

 signed to give fh-mness to the valve ; and its expansions are solid 

 nodules of the same substance. These nodules were mistaken by 

 Prof. Ehrenberg for apertures; and in this error he has been fol- 

 lowed by Kutzing. There cannot any longer, however, be a doubt 

 as to their real nature. As Prof. \Y. Smith has justly remarked : — 



