CHAPTER II. 



Terminology and Classification of Diatoms. 



We intend in this chapter to review the various parts of a diatom 

 with reference to the terms which we shall use in the classification 

 and description of species. We shall examine in succession the frustule, 

 the valve and its processes, the raphe, and the endochrome. 



i. Frustule. 



By frustule is meant the whole silicious box or epiderm. The frustule 

 is always formed of two parts called valves, one slipping over the other 

 or with edges opposed. , 



By girdle view is meant the view of the frustule which shews the 

 line of suture or self-division, called the connecting zone; and by valve 

 view that which presents the raphe fully or the punctate or striate 

 valve. 



The frustule is much developed in the girdle view when the axis 

 which is parallel to the sutural line is much shorter than the other. 

 The axis parallel to the sutural line is called the longitudinal axis and 

 the axis which forms a perpendicular with the sutural line is called the 

 transverse axis. 



The connecting zone may be broad or narrow ; it may be united 

 or display folds or puncta. 



The frustule is said to be geniculate or genuflexed when one of its 

 margins, or sometimes both margins, display a more or less sharp bend 

 as in the Ac/inantl/es (pi. 8, f. 323-337); it is called bent when the two 

 margins are curved alike and in the same direction, and lastly, it is 

 arcuate when one of its margins is more curved than the other. 



The frustule is said to be torsive or twisted when the two extremities 

 (apices) are turned in contrary directions : ex. Surirella spiralis (pi. 13, f. 592). 



It is called bacillar when it is much longer than it is broad, as in 

 Synedra (pi. 10, f. 402-438). 



The frustule is complex when it has several sets of valves as in some 

 Navicula, and it is compound when each valve consists of two or more 

 plates of different structure as in Arachnoidiscus and Actinopiychus (pi. 22, 

 f. 648, 649). 



