TERMINOLOGY AND CLASSIFICATION OF DIATOMS, 



95 



Table of the Pfitzer-Petit Classification. 



Order. Diatomaceae. -Unicellular alg?e with a cell (or frustule) 

 of two valves incrusted with silica, usually marked with very fine designs, 

 stria?, dots, etc. Endochrome brown or dark green. Reproduction by 

 conjugation and the formation of auxospores. Multiplication by division, 

 the new frustules capable of remaining united in a filament or separating 

 immediately. Cells free or adherent by a hyaline peduncle, or remaining 

 united in filaments after bi-partition or contained in a mucilaginous 



tube : 



Sub-order (A). PLACCOCHROMEiE. 



Endochrome arranged in more or less extended plates {Endochrome Laminate). 



Sub-order (B). COCCOCHROME^E. 



Endochrome arranged in isolated scattered grains, arranged spirally or in rays (Endochrome. 

 Granular ) . 



Sub-order (A). PLACCOCHROME^E. 



i. A single plate of Endochrome 



2. A plate of Endochrome, but perforated in the centre, and often 



so as to completely divide it into two portions 



3. Two plates of Endochrome ..... 



Sub-order (B). COCCOCHROME^E. 

 Frustules compound or J Valves linear, elliptic, constricted, cuneate 



with diaphragms. ( or conical 



Frustules simple . 



Ions 



\ 



' Valves elliptic or cuneate . 



Valves rounded, furnished with 



filaments ..... 



Valves polymorphous, triangular, quad- 

 rate, polygonal 



Free. Valves more or 

 less depressed 



United into cylindrical 

 filaments or two and 

 two. Valves more or 

 less convex like a 

 thimble 



Valves discoid, 1 

 Frustules : 



Sub-order (A.) PLACCOCHROME^E. 

 Family (I.) CYMBELLACE/E. 



Frustules with dissimilar valves ..... 



Valves cuneate, asymmetrical about the 



Frustules with 

 valves 



similar 



minor axis 



Valves cymbiform, asymmetrical about 

 the major axis .... 



Family (II.) NITZSCHIACE^E. 



f Valves linear, asymmetrical about one 



Frustules with similar j or both axes ; furnished with a punc- 



valves . . \ tate keel (Endochrome divided by a 



^ deep slit) ..... 



Families, 

 I. Cymbellacese. 



II. Nitzschiaceae, 

 III. Naviculacese. 



IV. Tabellariacese. 

 V. Fragilariacese. 



VI. Cb.se toceracese. 



VII. Biddulphiaceae. 



VIII. Coscinodiscacese. 



IX. Melosiracea?. 



Tribes. 



1. Achnantheaa. 



2. Gomphonemea?. 



3. Cymbellese. 



4. Nitzscliiese. 



