28 



Melosira. 



Melosira solida. Eul. in Y. H. Synops. T. 86, fig. 36 — 42. 



var. Sarsii, n. vai'. 



Tab. IV, fig. fil-66. 



Kjedernes diameter 7 — 12 ,«. 



Denne form findes i de fleste af Nordhavsexpeditionens 

 prøver og desuden i atlantisk plankton samlet af Nordgaard 

 i februar 96 og af Bie i mai 96. Den maa altsaa være 

 en regte saltvandsforni, medens den nærmeste slegtninge er 

 ferskvands- eller brakvandsformer. 



Thalassiosira, Cl. 

 Thalassiosira Nordenskioldii, Cl. (1873. 13) p. 6. T. II, fig. 1. 



Fig. V. H. Synopsis. T. s;i, fig. i). 

 Tab. IV, fig. .'.!). 



Cellerne er 12 — 43 u i diameter, i frontstilling næsten 

 ottekantede med et mindre antal sterke, i spidsen fortyk- 

 kede tagger langs skallens indre rand. Ved en central 

 slimtraad hænger de sammen i tætte, bugtede kjeder; mel- 

 lemrummene er sniallere end cellernes diameter (2 — 7 i.t). 

 Gjennem taggerne udgaar slimtraade i forskjellige retninger, 

 ligesom ogsaa undertiden hele kjeden kan omgive sig med 

 slim. 



Hvilesporer (Tab. IV, fig. 59) dannes, idet protoplas- 

 maet ti'ækker sig tilbage fra den ene skal tæt ind til den 

 anden og danner en ny skal indeni denne. 



Udbredélse. Atlanterhavet — Ishavet — Østersøen, ved 

 voi'e kyster især vinter og vaar. 



Thalassiosira gravida, Cl. (1896, 4) p. 12, T. n, 



fig. 14—16. 

 Tab. IV, fig. 57, 58, 



Cellernes diameter 17 — 62 ,«. Cellerne i frontstilling 

 rektangulære med afrundede hjørner, langs randen med 

 flere rader korte tagger, forenede i kjeder ved temmelig 

 lange, tykke, i midten indsnevrede slimstrenge. 



Hvilesporer beskrevne af Clere. 



Auxosporedannelsen er afbildet i)aa fig. 57 — 58, T. 

 IV. Cellerne fylder sig da med protoplasma, skallerne 

 viger fra hinanden, og protoplasmet omgiver sig først med 

 et tyndt, bøieligt hylle, deretter med to fastere skaller 

 indeni dette. Disse skaller er hvælvede, altsaa forskjeUige 

 fra de almindelige. Deretter begynder den nye celle, hvis 

 diameter er omtrent -f af den gamle, at dele sig, saa at 

 der dannes nye kjeder i forlængelsen af den gamle. I 

 almindelighed danner de fleste celler i kjeden omtrent sam- 

 tidig auxosporer. 



UdhredeJse. Atlanterhavet — Ishavet — Skagerak, ved 

 vore kyster især vinter og vaar. 



Melosira. 



Melosira solida, Eul. in V. H. Synopsis, Pl. 86, figs. 36—42. 

 var. Sarsii. n. var. 

 Pl. IV, figs. 64—66. 

 Diameter of filaments 7 — 12 ,«. 



This form is found in most of the North Atl. Exped. 

 samples, and also in Atlantic plankton collected by Nord- 

 gaard in February 1896, and by Bie in May 1896. It 

 must therefore be a true salt-water form, while those most 

 nearly related to it are fre.sh or brackish water forms. 



Thalassiosira, Cl. 

 Thalassiosira Nordenskioldii, CL (1873, 13) p. 6, PL II, fig. 1. 



Fig. V. H. Synopsis, Pl. 83, fig. II. 

 Pl. IV, fig. 59. 



Cells 12 — 43 j.t in diameter, in a front view almost 

 octagonal, with a small number of strong denticles, thick- 

 ened at tlie point, running along the inner margin of the 

 valve. They are connected by a central mucilaginous 

 thread in close, wavy chains; the intervals are narrower 

 than tlie diameter of the cells (2 — 7 (.i). Mucilaginous threads 

 run through the denticles in various directions, and some- 

 times the whole chain is enveloped in mucilage. 



Spores (Pl. IV, fig. 59) are formed by the drawing 

 back of the protoplasm from one valve close up to the 

 othei-, and the forming of a new valve within the latter. 



Distribiition. Atlantic Ocean, Arctic Ocean, Baltic; 

 on our coasts (Norway) partieularly in winter and spring. 



Thalassiosira gravida, CL (1896, 4) p. 12, Pl. II, 



figs. 14—16. 



Pl. IV, figs. 57, 58. 



Diameter of the cells, 17 — 62 u. Cells, in a front 

 view, rectangular. witli rounded angles, and several rows of 

 short denticles along the margin, united in chains by rather 

 long, thick mucilaginous cords narrowed in the middle. 



Spores described by Cleve. 



Auxospore formation figured in figs. 57 and 58, Pl. 

 IV. The cells are then filled with protoplasm, the valves 

 retreat from one another, and the protoplasm is enclosed 

 first in a thin, flexible investment, then in two finner valves 

 within this. These valves are convex. thns diftering from 

 the ordinary ones. Next. the new cells, whose diameter 

 is about ^ of that of the old ones, begin to divide, so as to 

 form new chains in continuation of the old one. Generally 

 most of the cells in the chain form auxospores at about 

 the same time. 



Distribuiion. Atlantic Ocean, Arctic Ocean, Skage- 

 rak; on our coasts (Norway) especially in winter and spring. 



