RHIZOSOLENIA CURVATA 419 



possible to work with very much larger fractions of the total catch than could have been 

 used if a complete analysis of the phytoplankton present had been aimed at. The only 

 samples with which it was necessary to examine fractions smaller than 1/300 were those 

 few where the phytoplankton was so heavy that even the large frustules of R. curvata 

 might have been obscured by the mass of other species present. The accessories used to 

 fractionize the samples were Stempel flasks, measuring cylinders, and Stempel pipettes, 

 all of the usual type. 



The microscope used was a monocular instrument of the usual type, fitted with a 

 large mechanical stage. The ordinary § and \ in. objectives were ample for the work in 

 hand. The fraction to be examined was spread over a definite rectangular area on a large 

 slide ruled in 2 mm. squares. Counting was effected by working up and down the rows 

 of squares, using a rather low-powered ocular, so that both sides of a square just entered 

 the field under low power, until the whole fraction had been examined. As each 

 frustule of R. curvata appeared, its diameter was measured under the higher power with 

 the aid of a micrometer eyepiece. No attempt to carry out a complete analysis of all the 

 phytoplankton present was made, but the more abundant forms or more obvious 

 dominants in each sample were noted. 



Single specimens or empty frustules of R. curvata were not considered adequate proof 

 of its presence at any given station. If only one individual was seen in a fraction, other 

 fractions of the same sample were worked through until at least five had been seen. In 

 this way it is hoped that the risk of basing records on fortuitous specimens from 

 previous net hauls has been minimized, although it is impossible to eliminate it 

 altogether. 



All the known positive records of the species, except some of our repeated series of 

 observations in longitude 8o° W, which lie too close together to be plotted clearly, are 

 given on the distribution charts (Figs. 1-4 and 7). The results obtained in longitude 

 8o° W have been plotted separately in Fig. 5. The tables deal only with the records for 

 which quantitative data have been obtained by the methods described above. In addition 

 to the estimated numbers of R. curvata per net haul, they show the station number, 

 date, and approximate distance in sea miles of each record from the average position of 

 the Antarctic convergence, and the surface temperature. In describing the relation of 

 the stations to the Antarctic convergence, it will be seen that the phrase "on the 

 Antarctic, or sub-Antarctic side of the convergence " has been used, instead of the more 

 obvious " north or south ". This has been done because the course of the convergence is 

 locally complicated ; its main component is not always east and west. For example, in 

 the Scotia Sea, in a very important region from the point of view of this study, it takes 

 an S -shaped course, resulting in a short projection of sub-Antarctic surface water east- 

 wards with Antarctic surface water both to the north and south of it. Surface tempera- 

 tures alone have been used because many of the observations are from intermediate 

 stations at which full hydrological data were not obtained. 



In order to obtain some idea of the seasonal variation in abundance of R. curvata, the 

 estimations have been grouped into month classes and averaged (Fig. 6). The data are 



