XVin INTRODUCTION. 



found those which are caused by the presence of Canahculi, to 

 which reference has already been made ; nor must we overlook 

 others, which, although connected with the structure of the valve, 

 arise from the absence of cells. Thus in many genera the valve 

 is notably distinguished by the presence of a longitudinal band 

 of silex, free from markings, and widened into smaU expansions 

 at the centre and extremities, or at the extremities only. This 

 band is simply a portion of the tissue in which the silex is de- 

 posited in a solid mass, and is probably designed to give firm- 

 ness to the valve. At the expansions of this band the silex 

 is still fiu'ther accumulated, and forms Nodules, which also 

 strengthen the epidermal envelope. 



That these expansions are not perforations in the valve, as 

 alleged by Ehrenberg, and acquiesced in by Kiltzing, might be 

 shown in various ways. The internal contents of the frustule 

 never escape at such points when the frustule is subjected to 

 pressure, but invariably at the sutm^e or the extremities, where 

 the Foramina, already described, exist. Nor does the valve, 

 when fractured, show any disposition to break at the expansions 

 of the central line, as would necessarily be the case were such 

 points perforations, and not nodules. Moreover, the central 

 band of silex is itself frequently traversed by a narrow line, which 

 arises from the confluence of a series of cells, which thus form a 

 minute tube ; but this tube invariably ends in a rounded ex- 

 tremity at the central and terminal nodules, and does not pass 

 into an opening or aperture in the valve. A reference to Plate 

 XIX. fig. 166. will illustrate these remarks. The bending down 

 of this tube, and the thickening downwards of the silex at the 

 nodules, give the semblance of depression to the surface of the 

 valve at such places. But I am disposed to think that this is 

 merely an optical appearance ; and am at all events assured that 

 no perforation exists at such points, and that the terms applied 

 to these nodules by different authors, implying that they are 

 openings or ostiola, are altogether inadmissible. 



