571 



the presence and absence of small chromatophores in the awns. 

 Judging from our present knowledge of the plankton-flora Ihere is 

 no doubt thai the main part of the plankton-diatoms has heen des- 

 cribed, hence I am of opinion thai the time has arrived when a 

 more extensive division of the genus Chaetoceras ought to be at- 

 tempted. Characters of systematic value are, in my opinion, the 

 following: — the number of the chromatophores; the form of the 

 valves and the hoop (girdle); the awns; and the endocysts, and I 

 think that the last-named character is one of the most important. 

 In the following I shall make an attempt at the division of 

 the genus, beginning with the two subgenera of Gran. 



Subgen. 1. PHAEOCERAS Gran. 



Sectio 1. Atlanticae. The directions of all the awns alwavs 

 alike (except the terminal ones); awns in one plane (sagitlal plane); 

 generally a small spine in the centre of the val ve. 



31. C. atlanticum Cleve, Arctic Sea, 1873, p. 11, PI. II, lig. 8 a, b; 

 C. dispar Castr., C. compactum Schutt. 



A boreal, oceanic species which occurs in the plankton all the year 

 round, but usually sparingly; common in the spring (March). Optimum 

 of temperature 4° — 5° C. [See tables.] 



Note. The following species belong to this section; — (l. neapolitanum 

 Schroder, C. skeleton Schutt, C. audax Schutt, C.dichaeta Ehbg. (— C.janischianum 

 Castr.), C. polygonum Schutt. 



Sectio 2. Boreales. Awns diverging in all directions; the direc- 

 tions of the awns of the one valve are often different from those 

 of the other valve; the small spine in the centre of the valve 

 absent. 



32. C. boreale Bail., Smithson. Contrib., 1854, p. 8, ligs. 22— 23. 



Found sparingly in the plankton gathered during most of the 

 months, rather common in some samples of May— July of 1898. Opti- 

 mum of temperature 7° — 10° C. t See tables]. 



C. boreale var. Brightwellii Cleve, Arctic Sea, 1873, p. 12, PI. II, 



lig. 7 a — e. 



Rare around the Færoes, found in a fcw specimens in March, 

 May August, November. 



33. C. convolutum Castr., Challenger Rep., 1886, p. 78, with tig.; 

 C. Brightwellii Gran., Protophyta, 1897, p. 11, PI. I, fig. 1 a— c, non 

 Cleve. Cfr. E. Jorgensen , Bergens Museums Aarbog 1900, No. VI, 

 pp. 20—22. 



