274 Bacillariece 



Sub-order 2. Solenoidece. Cells elongate, cylindrical or subcylin- 

 drical, circular or broadly elliptical in cross-section (or in the valve- 

 view) ; valves exhibiting a scaly structure ; apices often obliquely 

 conical and furnished with a spine or a hair. 



Sub-order 1. DISCOIDE^E. 



In this division of the Gentries the cells are more or less 

 shortly cylindrical and often disc-shaped. In the valve-view or in 

 cross-section they are circular, and the valve-faces are frequently 

 very convex. The cells may be solitary and free-floating, or they 

 may be joined into long chains or filaments by gelatinous cushions. 

 The valves are generally without any kind of protuberances, and 

 they may be hyaline or exhibit areolations or radial striations of 

 any degree of coarseness. Very often the markings on the valves 

 are divided into distinct sectors, and sometimes there is a ring of 

 small spines (rarely of long bristles) at the outer margin of each 

 valve. 



The chromatophores consist of numerous small plates, either 

 circular in outline or with lobed margins. The nucleus is generally 

 central, but it may lie near the girdle in a slight thickening of the 

 primordial utricle. 



Only two British families of this sub-order possess freshwater representa- 

 tives. 



Family 1. Melosiracece. Cells subspherical or shortly cylindrical, 

 circular in cross-section, and vmited to form long filaments or chains ; 

 girdle usually with a well-marked structure. 



Family 2. Coscinodiscacece. Cells mostly disc-shaped and solitary; 

 valve-view circular ; girdle usually without structure. 



Family 1. MELOSIRACE^l. 



The cells are mostly shortly cylindrical, rarely subspherical, 

 and they are usually united by gelatinous cushions to form 

 filaments of considerable length. The valve-view (and the cross- 

 section of the cell) is circular or very rarely somewhat compressed. 

 There is a great uniformity in the type of the cell, and the valves 

 may be flat, convex, or greatly arched. The valve-face is often 

 divided into concentric areas, a broad central region and a 

 peripheral ring of variable width. In some forms there is a 

 circular keel, occasionally provided with small wart-like protu- 

 berances, and in others the valves possess a peripheral ring of 



