84 TERMINOLOGY AND CLASSIFICATION OF DIATOMS. 



The frustule sometimes exhibits partitions or sepia, which are internal 

 longitudinal plates, that is to say, parallel to the connecting zone, as in 

 the case of the genera Rhabdonema (pi. 12, f. 486a-488a) and Striatella 

 (pi. 12, f. 483a-485a) ; these partitions are frequently (and most probably 

 always) perforated. 



In other cases the frustule shows false partitions or vittce; these are 

 prolongations from the margins of the connecting zone towards the interior 

 as in the genus Grammaiophora (pi. 11, f. 47g-482a). These imperfect 

 partitions are only visible in the girdle view, as may be seen in the 

 genus Meridian (pi. n, f. 474). 



The frustule is free when it is neither attached by any of its parts 

 nor enclosed in tubes or fronds ; it is adherent when it is attached 

 either to stones or to other algae or to any bodies by means of an 

 appendix produced by a prolongation of the coleoderm (mucous 

 envelope). 



In the latter case it is stipitate when this appendix forms a filament 

 of some length and sessile when its length is scarcely appreciable. 



Again, the frustule may also be enclosed in a mucus or in tubes 

 or in branched filaments resembling the higher algse, and in this latter 

 case these filaments are called fronds. 



The mucus, tubes, and fronds are formed of one and the same sub- 

 stance, viz., that which forms the coleoderm. This material, which is 

 hyaline, albuminoid, and slightly silicious, is sometimes called 

 thallus. 



The stipes, fronds, tubes, etc., are the result of an excessive develop- 

 ment of coleoderm, and this development varies with the habitat of the 

 species. Sometimes it is considerable, at other times it is entirely wanting ; 

 Mr. Kitton has found Navicula serians and Melosira Solerolii (which are 

 invariably free forms) enveloped in a thick mucus. 



Schizonema neglectum, Thwaites is nothing but Navicula gracilis, Kulz., 

 living in tubes. 



We do not therefore attribute any specific and less generic value to 

 these secretions of the frustule. 



2. Valves. 



A. General Description of the Valve. 



The valve is the moiety of the frustule. In comparing a diatom to 

 a box (e.g., pill box), the sides of the box correspond to the connecting 

 zone, and the valves to the top and bottom. 



