Bacillariece 273 



as the primary divisions adopted by Schiitt. The Bacillarieae are 



divided into the two following orders : 



Order I. Centricce. Valves with a concentric or radiating 

 symmetry around a central point ; without a raphe or pseudo- 

 raphe; valve-view circular, polygonal, or broadly elliptical, 

 rarely boat-shaped or irregular. 



Order II. Pennatce. Valves truly zygomorphic or irregu- 

 lar, never centric ; valve-view mostly boat-shaped or needle- 

 shaped, with markings arranged in a sagittal manner in 

 relation to the raphe or pseudo-raphe (sagittal axis). 



Order I. CENTRICLE. 



This order includes a relatively small proportion of the known 

 species of Diatoms, and few even of these are inhabitants of 

 fresh water. 



The cells are commonly cylindrical and when seen in cross- 

 section (or from -the valve-view) they are circular, polygonal, 

 elliptical, or rarely more elongate. Some forms are disc-shaped, 

 their diameter being much greater than their length ; others are 

 of equal length and breadth or longer than their diameter. Many 

 occur as solitary, free-floating individuals, but 'others are joined 

 more or less firmly by their valve-faces to form cylindrical filaments 

 of variable length. 



The structure of the valves is typically centric, even in those 

 species in which the valve-view is zygomorphic in outline. The 

 markings on the valve-face are either concentrically disposed or 

 arranged in the form of radiating striae, the latter often termi- 

 nating in marginal punctulations or dots. In all cases the 

 arrangement of the markings is in relation to a morphological 

 centre and never in relation to a middle line. There is no raphe 

 or pseudo-raphe. 



There are usually many small parietal chromatophores of a. 

 plate-like form in each cell, but occasionally only two large plates 

 are present. 



Auxospores are known only in a few species. 



The order is divided into four sub-orders, only two of which include 

 British freshwater species. One of the sub -orders the Biddulphioidece 

 includes a large number of conspicuous marine Diatoms. 



Svib-order 1. Discoidece. Cells shortly cylindrical or disc-shaped, 

 in valve- view circular ; hyaline or with radiating or areolated markings. 



w. A. 18 



