Rhizosoleniacece 279 



They are ornamented with several spiral lines, running along the 

 walls of the valves from end to end of the frustule and crossing one 

 another at intervals. The chromatophores are small and granular. 

 The structure of the cell- wall is only imperfectly known and the 

 position of the genus is uncertain. It should perhaps be associated 

 with the Nitzschiace*. 



C. gracilis (Bre"b.) Grun. \_ = Nitzschia Tcenia W. Sm.] is the only known 

 species and is a rare Diatom in the fresh and brackish waters of England. 



Order II. PENNAT^E. 



This is by far the largest order of the Bacillariese and includes 

 all those Diatoms in which the valves are not of a centric type. 

 The structure of the valves is arranged in relation to a line and 

 not to a central point. The cells are acyclic, being rod-shaped, 

 elliptic, or boat-shaped in cross-section (or valve-view). The 

 marks (striae or costse) on the valves are disposed on either side of 

 a sagittal line (which is either a raphe or a pseudo-raphe), some- 

 times arranged at right angles to it and sometimes forming an 

 acute angle with it. 



The cells exhibit great variability of form and in most cases are 

 truly zygomorphic. The commonest type of cell is the naviculoid 

 or boat-shaped type, but the frustule may be a flattened plate or 

 an elongated rod, which may be straight, arcuate, or sigmoid. 



The different forms exhibit all degrees of development of the 

 raphe. In the lowest forms of the Fragilarioideae it is entirely 

 wanting, but in other forms of the same group there are the first 

 beginnings of a pseudo-raphe. In the Naviculoidese the raphe is 

 present on both valves of the frustule, and it is sometimes situated 

 on a median or obliquely disposed keel. It reaches its highest 

 development in the Naviculaceae. 



There is a striking absence of spines and long processes from 

 the frustules of Diatoms of this order, although in a few instances 

 they are furnished with small prickle-like excrescences. 



Great variation is exhibited in the size and form of the chro- 

 matophores, which are in all cases parietal. In the lower families 

 of the Pennata? each cell contains a large number of small plates 

 (the coccochromatic arrangement), but in the higher families 

 one or few large lobed plates occupy the greater part of the inner 

 surface of the cell-wall (the placochromatic arrangement). 



