Achnanthacece 289 



In the first section, E. gracilis (Ehrenb.) Rabenh. (fig. 134 0) and E. pec- 

 tinalis (Kiitz.) Rabenh. are the two commonest species. In the second section, 

 E. lunaris (Ehrenb.) Grun. is much the commonest species (length of valves 

 50 90 p), but E. tetraodon Ehrenb., E. Veneris Kiitz. and E. biceps nob. 

 [ = Synedra biceps W. Sm. ; E. flexiwsa var. bicapitata Grun.] are general in 

 mountainous areas. 



Sub-order 2. ACHNANTHOIDE^E. 



The Diatoms of this sub-order are at once distinguished by the 

 crooked or geniculate character of the frustules. Seen in the 

 girdle- view each frustule is suddenly bent in its median portion, 

 a character not exhibited by any other group of Diatoms. Another 

 important feature of the group is the presence of a raphe on one 

 valve of the frustule and a pseudo-raphe on the other. Only two 

 families are included in the sub-order. 



Family 1. Achnanthacece. Frustules elongate and more or less 

 rod-shaped, generally occurring as stalked epiphytes. 



Family 2. Cocconeidacece. Frustules flat and plate-like, subcir- 

 cular or broadly elliptical in outline, occurring as epiphytes attached 

 by their flat valve-faces. 



Family 1. ACHNANTHACEJE. 



The frustules are straight and symmetrical in outline in the 

 valve-view, but geniculate and asymmetrical in the girdle-view. 

 In the valve-view they are in all cases linear or fusiform, often 

 with capitate apices. One valve possesses a pseudo-raphe without 

 any trace of nodules, whereas the other valve possesses a true 

 raphe with both central and terminal nodules. The valves are 

 transversely striated, the striae consisting of fine rows of punctae, 

 and in some species costse are present between the rows of punctae. 

 The chromatophore is usually a thick parietal plate, mostly covering 

 the convex valve, but in some species there are a number of small 

 granular chromatophores. 



Genus Achnanthes Bory, 1822. [Achnanthidium Kiitz., 1844 ; 



Grun., 1880.] The Diatoms of this genus possess the essential 



features of the family, and are either free-floating and solitary or 



stalked and epiphytic. There are two sub-genera, Euachnanthes 



w. A. 19 



