Classification 



121 



names in quite a different sense. As a classification it fails because of the 

 admittedly artificial group of the Cryptoraphidese. 



There are two classificatory schemes which have the advantage of being 

 much more natural than any of the others, and are therefore deserving 

 of more special mention. These are the schemes of Schiitt (1896) and 

 Forti (1912). 



The classification proposed by Schiitt ('96) has been adopted with slight 

 modifications by most investigators of the biology and taxonomy of diatoms, 

 and its great merit lies in the fact that all the principal groups are to a great 

 extent undoubtedly natural assemblages of forms. Its fundamental division, 

 separating all those diatoms with a radial symmetry of the valves (Centric 

 Diatoms) from those with zygomorphic or irregular valves in which the 

 structure is in relation to a longitudinal line (Pennate Diatoms), is sharp 

 and distinctive. It is briefly as follows : 



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Fig. 86. Two centric diatoms. The left-hand figure is the valve view of Aulacodiscus multipedex, 

 x 150 ; the right-hand figure is the valve view of Aulacodiscus Crux, x 120. 



A. Centricse. Valves with a concentric or radiating structure around a central 

 point ; without a raphe or pseudoraphe ; valve view circular, polygonal, or broadly 

 elliptical, rarely boat-shaped or irregular. 



a. Discoidex. Cells shortly cylindrical or disc-shaped, in valve view circular; 

 hyaline or with radiating or areolated markings. Includes the subdivisions 

 Coscinodiscete, Actinodiscese, and Eupodiscese. 



b. Solenoidese. Cells elongate, cylindrical or compressed, circular or elliptic in cross- 



section (or in valve view) ; valves often conical and furnished with a stout or 

 slender spine ; girdle complex, with a considerable development of intercalary 

 bands. Includes the Lauderiinse and the Rhizosoleniese. 



c. Biddulphioidese. Cells short, often a little longer than broad, box-like ; valves 



generally with two, or sometimes with more, poles, each pole with a hump-like 



