KONGL. SV. VET. AKADBMIENS HANDLINGAE. BAND 27. N:0 3. 



125 



Subgenus Oxyamphora. 



Frustule usually broadly elliptical, with truncate ends and complex connecting zone. Valves 

 lunate, acute. Median line approximate to the ventral margin. Central nodule transversely dilated 

 to a stauros or not. Axial and central areas usually indistinct. No longitudinal lines or keels 

 on the dorsal side of the valve. Structure of the dorsal side: usually fine puncta, arranged in 

 parallel, or slightly radiate, transverse striae, and in undulating, longitudinal lines. Ventral side 

 with, usually, much closer strise than the dorsal side. 



A. hyalina with cell-contents, frustule 

 in zonal view, in state of division, and 

 in valvular view; 600 times magnified. 



A. ostrearia with cell-contents ; 

 600 times magnified. 



Cell-contents. 1 have examined living specimens of A. hyalina, A. 

 In A. hyalina there is, along the ventral side of the connecting zone, a 

 the extremities of which are deeply indented and at the ends of which 

 a single elseoplast constantly occurs. On the dorsal side of the plate 

 and on its centre the nucleus occurs enclosed in a plasma-mass, from 

 which radiate fine threads towards the wall of the frustule. On divid- 

 ing, the plate and terminal elseoplasts are longitudinally cloven. If the 

 living frustule be stained with methylene-blue, numerous, intensely 

 coloured granules make their appearance in the primordial stratum of 

 the plasma. A. ostrearia has two chromatophore-plates, one along the 

 dorsal wall of the connecting zone. Both are deeply constricted in the 

 middlej A. quadrata has instead of chromatophore-plates a large number 

 of small, rounded discs, gathered below the central plasma-mass and at 

 the extremities. It is thus, as to its cell-contents, a coccochromatic 

 diatom. 



This subgenus comprises a number of forms with or without 

 stauros, which in other respects are too closely connected to be placed 

 in different sections. From the subgenus Ambly amphora they differ by 

 side and by the acute, frequently apiculate, ends of the valve. In the 

 respond, to Stauroneis and Libellus. 



ostrearia and A. quadrata. 

 flat chromatophore-plate, 



A. quadrata with cell-contents; 

 600 times magnified. 



the very narrow ventral 

 genus Navicula they cor- 



Artificial key. 



j Zone with some few inordinate stigmas A. magnifica Gebv. 



I — without — — — ■ "■ 



- ( Without stauros • •'• 



^- iwith - 10. 



( Thin and membranaceous ^■ 



\ Sillcious 6. 



