KONGL. SV. VET. AKADEMIENS HANDLINGAH. BAND 26. N:0 2. 5 



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

 are several forms, whicli owing to other importaiit characteristics must be placed in this genus, 

 althongli they are not at all gennilexed. Moreover gennflexed species occur in groups of allied 

 forms tlie greater number of which are straight, for instance in the Naviculce lineolata;, Nav. mi- 

 crostigmaticce, Gyrosigma, Fleurosigma, and in some species the degree of flexure seems to be subject 

 to variation (for instance in Gyrosigma arcficum). The geniculated flexure is tlius no generic cha- 

 racteristic, though in most cases of specific value. 



Zone. The zone in the Raphidies" is in most cases simple, that is withont longitudinal di- 

 visions, but in many Amphorce, in Amphiprora and in the group lAhellus of the Naviculce microstvi- 

 maticce 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 comnmtata seems in some varieties to have, and in others not to have, faint divi- 

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

 tudinal rows of alveoli on the zone. In the Naviculce microstigmaticce 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 ontline 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 o or 4 constrictions. 

 Sometimes the valve is sigmoid. 



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

 scriptions. 



4. 



Outline of the valves. Fig. 1 Linear, 3 Rliomboidal, 3 Lanceolate, 4 Rectangular, 5 — S Panduriform witli deltuid, sub- 

 elliptical, tongne-shaped and broadly rounded segments, 9 EUiptical, 10 Sigmoid, 11 Cymbiform, 12 Lunate, 13 Clavate. 



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



A 



1. 2. 3. 4. 5. 6, 7. 8. 



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 oi^tline is very diiferent in difPerent cases. Some species seem to be 

 very constant in shape, as Biploneis elliptica, many species of Pinnidaria, Cocconeis and Amphora. 



