GENERAL NOTES 209 



many intermediate forms exist. S. Palmeriana appears to favour tropical and subtropical 

 waters and a fairly high salinity. Stephanopyxis turris, on the other hand, frequents 

 temperate waters, is much smaller in diameter, stouter in structure and usually con- 

 stricted in the valve mantle. 



Under the subfamily Thalassiosiroideae are grouped together two truly planktonic 

 sections of diatoms, one mainly inhabiting temperate and cold waters, the other pre- 

 ferring subtropical waters of fairly high salinity and a high hydrogen-ion concentration. 

 In the first section, Thalossiosira of Cleve is the most important, six species being re- 

 corded ; T. antarctica Comber and T. subtilis (Ostenfeld) Gran frequently occurred in 

 enormous colonies. The former exhibited much variation in size and coarseness of 

 marking and enjoyed a very wide distribution. Auxospores and resting spores were 

 observed in some of the species. 



The genus Schroderella of Pavillard is the most important of the second section ; two 

 species were recognized, namely, S. delicatula and S. Schroderi. These have been com- 

 bined by some authors, but from the material under consideration no facts emerged 

 that would warrant such a course being taken. S. delicatula was abundant at some 

 stations off the Cape, while S. Schroderi was frequently observed in the Peru current. 



In the subfamily Coscinodiscoideae two main groups are again recognized ; first, the 

 complex genus Coscinodiscus and Charcotia and Actinocyclus, which are in the main 

 heavy forms (some are truly planktonic, but many are bottom forms); and secondly, 

 what might be described as the "levigated discoids", including Planktoniella , Valdi- 

 viella, Gossleriella, etc. 



In the genus Coscinodiscus the analysis adopted by Rattray (1890) has been used. This 

 is based on the disposition of the markings upon the valve surface and is as follows : 



Inordinatae : forms with markings usually granular, and arranged irregularly. 



Excentricae: forms with areolate markings which do not radiate from the centre of the valve; 

 areoles in slightly curved and nearly parallel rows. 



Lineatae: forms with markings usually areolate, which run across the valve in straight lines. 



Fasciculatae : forms with areoles forming sectors; areoles in parallel lines. 



Radiatae: forms with areoles in more or less straight lines radiating from the centre of the valve. 



Stellatae: forms with a central stellate group of nodules or thickenings; markings very fine, usually 

 radiate. 



Nearly forty species have been recorded, the majority being found in temperate or 

 cold water. The heavy forms were found at no time in great quantities, but occurred 

 chiefly in samples where the net had touched bottom. 



Those species which are to be considered true members of the plankton occurred 

 occasionally as almost pure gatherings. The chief among these being Coscinodiscus 

 centralis, C. sub-bulliens, and C. bouvet. The first two are closely allied species and are 

 frequent members of northern plankton, though C. bouvet has not yet been recorded 

 from northern waters. C. kerguelensis might also be mentioned as a truly planktonic 

 form which occurred in fair quantities. All of these are subpolar forms. The genus was 

 represented in the plankton of warmer waters mainly by C. gigas and C. concinnus ; both 

 favour a high salinity and are weakly siliceous. 



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