262 



16. D. didyma Ehr., V. H. Syn. Suppl. B, fig. 20. 

 Trangisvaag (Sy der 6). 



17. D. elliptica Kutz., A. S. Atl. tab. VII, figs. 29 and 32, V. H. 



Syn. tab. X, fig. 10. 



Found dispersed in most of ttie gatherings, but never in consider- 

 able quantity. (Plankton). Varies somewhat in size. In gatlierings 

 from Hojefjæld (Bord 6) and Viderejde (Vider o) I liave found smaller 

 forms, which agree most closely to D. Paella (Schum.?) Cl. V. H. Syn. 

 tab. X, tig. 11, and are undoubtedly only variations of D. elliptica (cfr. 

 Cl. Syn. I, p. 92). 



18. D. ovalis Hilse., A. S. Atl. tab. VII, fig. 33. 



Not rare in a gathering, labelled: »Algæ, in slowlj^ running water 

 near Bosdalafos« (Vaago). 



19. D. Smithii Bréb., Grun. Vz. Jos. L. tab. I, fig. 41. 

 Trangisvaag (Sy de ro). 



GYROSIGMA Hassall 1845. Cl. Syn. 1, p. 112. 



20. Gyposigma acuminatum Kiitz., V. H. Syn. tab. XXI, fig. 12. 

 Rare. Tværaa (Sy der 6) and in Plankton from the lake in Vaags 



Ej de (Vaago). 



FRUSTULIA Ag. 1824. Cl. Syn. I, p. 122. 



21. Frustulia vulgaris Thw., V. H. Syn. tab. XVII, fig. 6. 



22. F. rhomboides Ehr., V. H. Syn. tab. XVII, figs. 1-2. 

 Var. saxonica Rbh., V. H. 1. c, fig. 4. 



Var. viridula Bréb., V. H. 1. c, fig. 3. 



The above-mentioned species and variations of Frustulia occur rather 

 often in the material. F. rh. saxonica appears to be the most frequent, 

 it, however, varies in having apices more or less capitate. 



23. F. vitrea m. Fig. 30 ^^^/i. 

 L. 0,02— 0,024 mm., B. 0,0055 mm. Valve nearly 



linear; apices truncate. Central pores comparatively 



distant. Striæ exceedingly delicate, just visible on 



applying Zeiss' Apochromate 2 mm.; they appear to 



Fif 30. be at right angles to the raphe. I think I have seen 



two longitudinal lines close to and parallel with the margin, but I 



am not quite sure of it. 



The reason why 1 have placed this small hyaline form under 

 Frustulia is mostly on account of the appearance of its central nodule. 

 It may perhaps be related to Navicula El Kab O. M. (Bacil. a. d. Natron- 



