290 



Schiitt, 



Bacillariece 

 \ 



in which the raphe and pseudo-raphe are straight and 

 median or a little excentric; and Ach- 

 nanthidium Kiitz., in which the raphe 

 and pseudo-raphe are sigmoid. In some 

 species there is a distinct stauros on 

 the lower valve. Auxospores may be 

 formed in two ways : two from a mother- 

 cell without conjugation, or one formed 

 by the conj ugation of two cells. 



The largest freshwater species with a straight 

 raphe, and also rather a rare one, is A. coarctata 

 Brdb. ; length of valves 18 43 /*. A. exilis 

 Kiitz., A. linearis W. Sm. and A. microcephala 

 Kiitz. are very small species which are often 

 abundant, frequently occurring as stalked epi- 

 phytes in thick masses round other filamentous 

 Algae. A. flexella, (Kiitz.) Brdb. possesses a 

 sigmoid raphe and often occurs in abundance 

 in hilly districts ; length of valves 35 50 p.. 



Fig. 135. A C, Achnan- 

 thes Hungarica Grun., from 

 near Sutton, Cambridge ( x 

 520). D F, Cocconeis Pla- 

 centMZaEhrenb.,from Sheep's 

 Green, Cambridge ( x 520). 



Family 2. COCCONEIDACEJE. 



The frustules are symmetrical in both the valve-view and in 

 transverse section, as in the family Achnanthacese. They are, 

 however, flat and plate-like, exhibiting a broadly elliptical or 

 subcircular outline in the valve-view. The valves are dissimilar, 

 one possessing a pseudo-raphe and the other a raphe with a central 

 nodule, but polar nodules are generally absent. The frustules 

 frequently possess perforated transverse septa which appear in the 

 valve-view as ribs. The girdle-view is generally more or less bent 

 or arched. The cells possess one chromatophore, which is mostly 

 parietal on the convex valve. 



Genus Cocconeis Ehrenb., 1835. [Orthoneis Grun., 1868 ; 

 Anorthoneis Grun., 1868.] This is the only genus of the family 

 containing any freshwater species. The frustules are flat, plate- 

 like or leaf-like, and somewhat the shape of a concavo-convex lens ; 

 they possess punctate striae, transverse in the middle of the valve 

 and radiating towards the poles. The cost* seen in the valve-view 

 of some species and which represent the false septa, are confined 

 to the outer border of the valve. One auxospore is formed from a 

 single mother-cell. 



