Aquatic JLttt 



93 



longitudinal line in the other. In a view 

 at right angles it is narrow, quadrangu- 

 lar, and slightly bent in the middle as is 

 the case with Achnanthes exigua, It is 

 sometimes more abundant than any other 

 form. (Fig. 4.) 



Cocconcis placentula. This form is 

 broadly elliptical and flat, like a little cake, 

 as its name implies, and the two valves 

 are not alike, one having a distinct longi- 

 tudinal line and a well marked central 

 space, while the other is without the cen- 



1. Navicula minima. 2. Eunotia pectinalis minor. 3. Achnanthes exigua. 4. 

 Achnanthes linearis curta. 5. Gomphonema angustatum. 6. Cocconeis placentula. 

 7. Nitzschia amphibia. Drawings by author. 



Gomphonema angustatum. The out- 

 line of this form is lanceolate but one end 

 is broader and shorter than the other, 

 with a blank space in the middle nearer 

 the broad end. In the other view it is 

 slightly cuneate. The length is three 

 times that of the Navicula, (Fig. 5.) 



tral space. Sometimes a rim surrounds 

 one of the valves divided into minute 

 cells. (Fig. 6.) 



Nitzschia amphibia. Nitzschiae differ 

 from other diatoms in that one edge is 

 much more definite in marking than the 

 other and in this little form, one of the 



