280 Bacillariece 



The methods of formation of auxospores are as yet only 

 imperfectly known. In some of the higher forms the method 

 is a sexual one, and the highest known type is found in the 

 Surirellacese. 



The order is divided into the following sub-orders : 



Sub-order 1. Fragilarioideoe. Cells mostly straight, rod-shaped or 

 lanceolate, without a raphe, but sometimes with a pseudo-raphe or 

 showing indications of the commencement of a raphe. 



Sub-order 2. Achnanthoidefe. Cells crooked or suddenly bent, 

 with a raphe on one valve and a pseudo-raphe on the other. 



Sub-order 3. Naviculoidece. Each valve of the cell with a raphe ; 

 valves without a keel (or rarely with a keel) in the sagittal line (line of 

 raphe). 



Sub-order 4. Nitzschioideoe. Each valve of the cell with a raphe, 

 which is situated in a sagittal keel with carinal dots. The keels of the 

 two valves are situated on opposite sides of the frustule or displaced 

 to the same side. Cells in transverse section rhombic. 



Sub-order 5. Surirelloidece. Each valve of the cell with a pseudo- 

 raphe and generally with spreading submarginal wings ; valves strongly 

 costate. 



Suborder 1. FRAGILARIOIDE^. 



In this sub-order the cells are mostly rectangular in the girdle- 

 view, and rod-shaped, lanceolate, or arcuate in the valve-view. In 

 the Meridionacese they are elongate and cuneate in both views. 

 Sometimes they occur as solitary individuals, but they are more 

 commonly joined by gelatinous cushions into either straight or 

 zig-zag, ribbon-like filaments. In some there are false septa 

 traversing the frustules, but in others these are absent. There is 

 no raphe, but in most of them there is a pseudo-raphe forming a 

 sagittal line in relation to which the markings of the valves are 

 arranged. In the Meridionaceas the presence of this pseudo-raphe 

 is scarcely evident. The chromatophores are mostly numerous, 

 small and granular, but in the Eunotiacea3 they are few in number, 

 large and plate-like. 



The sub-order includes the following five families : 



Family 1. Tabellariacece. Cells forming tabular plates in the 

 girdle-view, united to form band-like or zig-zag filaments; with well- 

 developed false internal septa. Valves with a straight median pseudo- 

 raphe. 



