29 



Thalassiosira Clevei, n. sp. 



Tab. IV, tig. till— 6-2. 

 I nogle planktonprøver, samlede af Haslum i det nord- 

 vestlige Atlanterhav paa omtrent 66" N. br. og 30" L. W. 

 findes i store mrengder en Thalassiosira, der er forskjellig 

 fra Th. gravida, med hvilken den forekommer sammen. 



Cellerne er skiveformede, i frontstilling fladt rektangu- 

 lære med budte hjørner, IG..^^— 42 ^i i diameter, ved en 

 central slimstreng forenede i bugtede kjeder med uregel- 

 mæssige intervaller, snart 2 — 4 eller flere celler tæt sam- 

 men, snart et bredere mellemrum med en ganske tynd for- 

 bindende traad. 



Skallerne bærer i randen eu enkelt tæt række af 

 korte tagger, 6—7 paa 10 ^/. Paa et enkelt sted desuden 

 en grovere knude lidt indenfor taggernes cirkel ; disse knu- 

 der, der paa de to tilsvarende skaller staar diametralt 

 modsat, synes at danne kanaler for slimtraade. 



Skallernes masker omtrent som hos TJ/, graridaj paa 

 de af mig undersøgte exemplarer 13 — IG paa K.) /(. 



Prøverne er tagne om sommeren, og derfor er alle 

 arters cellevægge tynde, svagt forkislede, og skallernes 

 struktur er vanskelig at se; Th. Clevei er nær beslegtet 

 med Coscinodisms kryophilus, Grun. (1884) Pl. III, fig. 21, 

 maaske vil den vise sig at være identisk med denne art. 



Hvilesporer og auxosporer ukjendte. 

 ForcJiOjiisf. N. V. Atlanterhav (Haslunt), Lofoten 

 marts — april 9lj {Nordgaard). 



Coscinodiscus, Ehv. 



Af denne slegt er der efter skallernes struktur be- 

 skrevet et meget stort antal arter. Naar disse arter bliver 

 nærmere kjendte i levende tilstand, vil visselig opfatningen 

 af deres systematiske forhold blive meget forandret, og 

 slegten vil ved biologiske karakterer blive opdelt i flere nye 

 slegter. 



En begyndelse hertil er skeet ved oprettelsen af sleg- 

 ten Tlialassiosira, en adskillelse, som efter min mening er 

 vel berettiget. Til denne slegt vil sandsynligvis ikke faa 

 af de nuværende Cosci nodisms-avter maatte henføres. 



I det følgende har jeg opfort to arter, som udcn tvil 

 burde udskilles fra slegten; men da jeg endnu kun har 

 bavt anledning til at undersøge et lidet antal arter, vil jeg 

 forelobiii henfore dem til Coscinodiscus. 



Thalassiosira Clevei, n. sp. 



1'1. IV, tigs. liU— (i'i. 



In some plankton sainph^s coliected by Hashtiii in 

 tbe north-western Atlantic, in aliout 66" N. Lat. and 30" 

 W. Long., a Tha1a.mosira is found in great quantities, 

 whicli is diftcrent to TJi. gravida, in whose company it is 

 found. 



The cells are lamelliform, in a front vicw of a flat 

 rectangular shape, with blunt angles, Ifi..") — 42 /< in dia- 

 meter, connected by a central mucilaginous cord in wavy 

 chains witli in-cgulai- intervals, soraetimes from 2 to 4 or 

 more cells close together, sometinies a widei- interval with 

 quite a thin connecting thread. 



The valves carry at tlic margin a single, close row 

 of short denticles, 6 or 7 in 10 u. Tiiere is moreover, at 

 ono place, a larger protuberance a little within the circle 

 of denticles. These protuberances, which are diaraetrically 

 opposite to one anotlier on the two corresponding valves, 

 appear to form channels for the mucilaginous threads. 



Tiic striæ of the valves almost as in Tii. gravida; 

 in tiie si)ecimeus I examined, there were from 13 to 16 

 in 10 (,. 



The samples vvere tåken in the summer, and tliere- 

 fore the cell walls of all tlie species are thin and sligiitly 

 siliceous, and it is difficult to see the structure of the 

 valves. The species is nearly allied to Coscinodiscus hryo- 

 philus, Grun. (1884) Pl. III, fig. 21; it may possibly be 

 identical with it. 



Spores and auxospores unknown. 



Occurrence. X. W. Atlantic Ocean (Haslum); Lofoten, 

 March, April 1896 (Nordgaard). 



Coscinodiscus, Ehv. 



A very large number of species of tliis genus iiave 

 been described from the structure of the valves. When 

 these species become better kuowii in a living state, opinions 

 concerning their systematic conditions vvill certainly be 

 greatly changed, and the genus divided by biological cha- 

 racters i uto several new genera. 



A step towards this has been made by the establish- 

 ment of the genus Thcdassiosira, a separation which, in 

 my opinion, is thoroughly justifiable. It is probable that 

 no small number of the present Coscinodiscus species will 

 have to be referred to Thalassiosira. 



I give below two species which ought without tloubt 

 to be €eparated trom the genus; but ha ving, as yet, only 

 had the opportunity of examining a small luimber of spe- 

 cies, I will in tile mean time refer them to Coscinodiscus. 



