DIATOMACE^E 



Unicellular or filamentous. Cells either free, sessile, united in filaments, immersed in a 

 gelatinous envelope or in fronds composed of branching tubes; microscopic, enclosed in a 

 more or less siliceous envelope (frustule), composed of two parts (valves), usually connected 

 by an intervening band (zone or girdle). Cell contents include yellowish or brownish chloro- 

 phyll-like bodies which occur in one or several bands (placcochromatic), or as variously dis- 

 tributed granular masses (coccochromatic) lining the inner walls. Growth by ordinary cell 

 division or by auxospores; sexual multiplication by the formation of sporangia. Valves of 

 two kinds : (a) Those in which the markings or parts are more or less concentric 

 (6) Those (Peimatae) in which the parts are more or less symmetrically divided By a line 

 (pseudoraphe) o* by a cleft (raphe). 



CENTRIC.E 



Valves without a dividing line or cleft; markings more or less radiate; transverse section 

 of frustule circular, polygonal, or elliptical, sometimes irregular. 

 Divided into four groups: 



1. Discoidtce. Frustules (cells) discoid; valves without horns or elevations (sometimes 

 with processes). 



2. Soknoidece. Frustules with numerous girdle bands. 



3. Biddulphioidece. Frustules box-like, i. e., with the longitudinal axis greater than in 

 the Discoidese. Valves with two or more angles, elevations or horns. 



4. Rutilarioidece. Valves as if naviculoid, but with irregular or radial structure. 



Groups 2 and 4 are not included in our description. No. 2 contains plankton genera 

 only, while No. 4 consists of genera not yet found in this locality. 



DISCOIDE.E 



1. Costinodiscea. Valve not divided by rays or costae into sectors; puncta sometimes 

 radiate; ocelli or processes absent. 



2. Actinodiscece. Valve with radial striae divided into sectors ; ocelli and processes absent. 



3. Eupodiscece. Valve disc-shaped with mammiform processes or one or more ocelli. 



1. COSCINODISCE.E 



(a) Meloseirince. Frustules short, in chains. 



(b) Coscinodiscince. Frustules disc form, usually single, rarely in short chains. 



(a) MELOSEIRIN.-E 



1. Meloseira. Valve punctate, with a constriction or furrow between edge of valve and 

 girdle. 



2. Gaillonella. Valve punctate, with a circular collar or crest near edge of valve. 



3. Lysigonium. Valve punctate, neither keeled nor constricted. 



4. Hyalodiscus. Valve punctate in the centre; border with decussating radial lines. 



5. Stephanopyxis. Border of valve with a crown of thorns; valve areolate. 



6. Pyxidicula. Valve areolate, with a border of spines. 



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