Terms Referring to the Structures of Diatom Frustules 



(Numbers refer to the illustrations; plan of diagrams (p. 940) borrowed 

 from Fritsch. ) 



1. Frustule: the Diatom cell; the shell. 



2. Epitheca: the older, larger portion of the shell fitting over the smaller. 

 2', Epivalve. 



3. Hypotheca: the younger, smaller portion of the shell; 3', Hypovalve. 



4. Connecting Band: the rim of the hypotheca or epitheca; rim of the 

 valves. 



5. Girdle: the section composed of the connecting bands which lock the 

 two portions of the wall together. 



6. Valve View: view of the frustule as seen from top or bottom. 



7. Girdle View: view of the frustule as seen from the side so that the 

 girdle area is in view and when the overlapping of the two valves is 

 visible. 



8. Polar nodule: internal wall thickening of the valve near the poles. 



9. Central nodule: internal wall thickening of the valves at the center. 



10. Raphe: a well-marked line formed by a canal which runs through the 

 top or bottom walls of the valve, connecting the polar and central 

 nodule. 



11. Pseudo raphe: a narrow, linear area in the mid-line of the valve which 

 is smooth, i. e. contains no wall markings and superficially appears as 

 a raphe. 



12. Intercalary Bands: hoop-like, secondary connecting bands which are 

 the incurved edges of the valve and which are attached to the connect- 

 ing bands. 



13. Septa: incomplete partitions running parallel with the valves and 

 which result from internal extention of the intercalary bands. 



14. Valve Markings: 



a. Striae-linear markings, sometimes actually composed of closely spaced 

 puncta ( points ) . 



b. Puncta: minute points or pits in the wall, usually occurring in rows. 



c. Costae: conspicuous ribs, double lines, actually tubular structures 

 in the wall. 



A Key to the Common Genera of Freshwater Diatoms 



1. Frustules elongate, rod-shaped, boat-shaped, rectangular or wedge- 

 shaped, two or more times longer than wide 14 



1. Frustules isodiametric or nearly so; round, triangular, or oval, but 

 less than twice the diameter in length 2 



2. Frustules, rectangular in side or girdle view, joined in chains by 

 interlocking of long, slender, spine-like horns which arise from the 

 corners of the valve; frustules without raphe; horns hollow or 

 solid Chaetoceros 



PL 5, Fig. 1 



2. Frustules without spine-like horns 3 



3. Frustules triangular in valve view; seldom seen lying in girdle view; 

 raphe and pseudoraphe lacking Hydrosera 



PI. 5, Fig. 3 



3. Frustules some other shape 4 



[942] 



