KONGL. SV. VET. AKADEMIENS HANDLINRAK. BAND 26. N:0 2. 



form of the objects, to which these forms are attached by the lower valve, and in Achnanthes there 

 are several forms, which owing to other important characteristics must be placed in this genus, 

 although they are not at all genuflexed. Moreover genuflexed species occur in groups of allied 

 forms the greater number of which are straight, for instance in the Naviculee lineolatce, Nav. mi- 

 crostigmaticcB, Gyrosigma, Pleurosigma, and in some species the degree of flexure seems to be subject 

 to variation (for instance in Oyrosigma arcticum). The geniculated flexure is thus no generic cha- 

 racteristic, though in most cases of specific value. 



Zone. The zone in the Eaphidieie is in most cases simple, that is without longitudinal di- 

 visions, but in many AmpJiorce, in AmpMprora and in the group Libellus of the Naviculee microstig- 

 maticm it is complex. This characteristic appears to be subject to very little variation and to be of 

 importance in the limitation of genera. There are however in some cases small variations. The 

 zone of Amphora commutata seems in some varieties to have, and in others not to have, faint divi- 

 sions. Amphora rohusta has usually no divisions, but in a variety from California there are longi- 

 tudinal rows of alveoli on the zone. In the Naviculee microstigmaticee also, some allied forms have 

 divisions and others are without them. Still we may consider the complexity of the zone as an 

 important characteristic. 



Outline of the valves. The outline of the valves of the naviculoid diatoms is very variable, 

 presenting every intermediate passage from narrow linear to almost orbicular. The valves are 

 non-constricted, constricted, or biconstricted, and, occasionally, some forms have 3 or 4 constrictions. 

 Sometimes the valve is sigmoid. 



The following schematic figures will illustrate the meaning of the terms used in my de- 

 scriptions. 



1. 



[" 



~^ 



6. 



'A 



Outline of the valves. Pig. 1 Linear, 2 Rhomboidal, 3 Lanceolate, 4 Rectangular, 5 — 8 Panduriform witli deltoid, snb- 

 elliptical, tongue-shaped and broadly rounded segments, 9 Elliptical, 10 Sigmoid, 11 Oymbiform, 12 Lunate, 13 Clavate. 



The ends of the valves also have a great variety of shapes, as 



/\ 



1. 



2. 



5. 



Ends of the valves. 1 Acuminate, 2 Apiculate, 3 Rostrate, 4 Capitate, 5 Truncate (subrostrate), 6 Cuneate, 7 Rounded obtuse, 



8 Obliquely or unilaterally rounded. 



The constancy of the outline is very different in different cases. Some species seem to be 

 very constant in shape, as Diploneis elliptica, many species of Pinmilnria, Cocconeis and Amphora. 



