Protistplankton. 



91 



Such species are noted as native, or sonietinies as .,stationai'y", on 

 our coasts. 



So as not to ditler more tiian necessary from tlie expressions 

 commonly used, I iiavc called boreal those coast forms which are 

 stationary on our northern coasts, as well as those which, according 

 to my opinion, come to us from the northern, but not exactly the 

 arctic, districts. (This expression — boi-cal — was first used by 

 Cleve, and later by both Gkan and Ostenfeld with a similar 

 meaning). Moreover, 1 have, as Gran has done, widened this 

 expression to include certain oceanic forms, which are found in the 

 northern Atlantic outside tiio arctic water proper. After my view, 

 such forms will for the most part be those which thrive in the 

 boundary lines between the ai'ctic water and that of the Atlantic, 

 and which are well able to bear the latter (up to a certain degree 

 of salinity and temperature). 



That it is often difficult to decide whether arctic and boreal 

 (I would prefer to call the latter subarctic) forms are neritic or 

 oceanic, is something which is a necessary result of my opinion 

 that they thrive well and may give rise to evolution en masse 

 in the boundaries between the arctic and Atlantic waters. It is 

 this fact which has also been referred to by some authors when 

 saying, that these boundaries to some extent play the same part as 

 the coasts. 



Co^einodi><<>iiK Ehrb. 



It will be seen, from the various plankton tables which have 

 been published, that this difficult genus has given rise to much 

 confusion. The names which are used in many cases evidently 

 mean quite diffei-ent species. As there, however, in our latitude, 

 does not appear to be very many species in the plankton, it ought 

 to be possible to arrive at comparative clearness concerning them. 

 It is probable that in reality there are many more species than 

 have up to the present been found : but there are only a few which 

 occur frequently. 



I -nill here give a brief survey of the species which I have 

 mentioned in the plankton tables. 



Key to the .■specie» of CoscnioiViscn^. 



Valve flat or nearly so (sometimes suddenly descending at a nar- 

 row zone of the outmost margin). 



Marginal apiculi present (always distinct). 



The characteristic structure of 

 C. curvatulus: valve by (somewhat 

 curved) radii di\1ded into a con- 

 siderable number of sectors; mar- 

 kings (areoles) in each sector in 

 rows parallel to the one limiting 

 radius (or somewhat convergent to- 

 wards the border) C. curvatulus. 



The characteristic structure of 

 C', lineatus: markings arranged in 

 more or less straight rows (in 5 or 

 6 directions) across the valve .... C. lineatus. 

 (et var.) 



The characteristic structure of C. 

 excentrieus : markings in 7 fasciculi, 

 forming distinct secondary curves, 

 concave towards the border C. excentrieus. 



Fine radiating structure with 

 more or less distinct and numerous 



fasciculi C', hioculatus. 



No marginal apiculi. 



Structure of the valve rather 

 coarse, more or less distinctly radi- 

 ate (only near the border with more 

 or less visible fasciculi, consisting 

 of rows converging towards the 

 border, sometimes apparently inordi- 

 nate); markings near the border sud- 

 denly much smaller C. radiatus. 



Valve decidedly convex (in C', decipiens witli liiL'h 

 broad marginal zone, then flat). 



Valve without close markings all ovei' tlie 



surface, only with distant jiuncta. C', nitidus. 

 Valve with close (more or less distinctly 

 polygonal) markings. 



Fine structui'e (fasciculi) ; around 

 the centre of the valve a con- 

 spicuous star consisting of about 



5 coarse marks C', stellaris. 



Structure similar to that of C'. 

 excentrieus, but with more quickly 

 diminishing markings and long mar- 

 ginal apiculi C. decipiens. 



(= Thcdassiosira gelatinosa). 

 Structure radiate, with more or 

 less branched radii. No coarse 

 marks ; marginal apiculi (when pre- 

 sent) short and small. 



2 comparatively large (short 

 linear) asymmetrical and 

 numerous small marginal api- 

 culi (which are often very 

 inconspicuous or apparently 

 wanting). 



Fine struetui-e. Smooth 

 central space or large 

 central rosette of se- 

 veral times larger are- 

 oles. Valve thin C. eoneinnus. 



Coarser structure. At 

 the centre a definite 

 central rosette of doub- 

 ly large areoles. More 



strongly siliceous C. centralis. 



No marginal apiculi. 



Valve veiT thick. 

 Coarse structure; are- 

 oles of very varying 

 size on one and the 

 same valve, from -/s 

 of the radius some- 

 what smaller. Com- 

 paratively broad, co- 

 arsely striated, border C', siibbxdliens. 



