SYSTEMATIC ACCOUNT 327 



even gradation and that as far as measurements were concerned the phases merged into 

 one another as a continuous chain. The adjacent selection of dimensions gives some idea 

 of the variation met with. The dimensions (in microns) are arranged as : diameter of 

 cell x pervalvar axis. 



The names chosen to describe the phases are placed in order of priority. In order to 

 render the distribution more clear, the stations at which all phases of the Corethron 

 population occurred are quoted below together, and the distribution is expressed in 

 general terms under the phase titles. 



Observed at Sts. 300-305, 334-340, 365, 368, 378-388, 433, 436, 439, 450-453, 460, 

 461, 463, 475, 477-483, 5 OI -5 o8 > 5^S l 3> 542-544, 55^553, 574~578, 580, 615, 617, 

 619, 626, 661, 663, 664, 666, 670, 1356, 1358, 1359, 1362, 1369; WS 100, 101, 106, 

 469, 474, 481, 540-543, 545, 547-552A, 569, 571, 593, 594, 598, 600-602, 621-623, 

 640, 644, 703, 705, 706, 710; MS 86, 94, 97, 99; RS 9, 17, 19, 20, 23, 27. 



"criophilum" phase. (PI. VII, figs. 2-10; pi. VIII, figs. 3-6.) 



This might be described as the type phase, and to take Castracane's own words, 

 " this long and perfect little cylinder has a longitudinal axis, which bears to its diameter 

 the ratio of 14 to 1. The awns are long and very delicate, smooth and radiating in the 

 same direction at the two extremities. The two valves are extremely convex". 



The illustration provided by Castracane shows a plain, hyaline, thin-walled cell, 

 provided at both ends with a circlet of long bristles surrounding the deeply convex 

 valves. The valves are sometimes constricted in the valve mantle. This thin-walled phase 

 is probably a summer form or a special form produced from microspores. It was ob- 

 served in the Atlantic from time to time, particularly at Sts. 664, 666 and 670 upon the 

 30th W meridian, where the specimens compared exactly with Castracane's description 

 and illustration. It was observed in very large numbers in the Ross Sea material but 

 here the cells were exceedingly small and very weakly siliceous. In this area also, it was 

 observed that Corethron adopts the colonial habit and large numbers often were en- 

 countered embedded in a mucilaginous film. These colonies were of considerable size, 

 often as many as several hundred cells being so united. The mucilaginous groundwork 

 of the film would assume a pale purplish-blue colour when stained with methylene blue, 

 and large numbers of small granules were embedded in it which absorbed the dye very 

 readily, assuming a blue-black colour. The small cells embedded in the film absorbed 

 the dye but weakly, assuming a pale blue colour, but the cytological elements within 

 them were stained very darkly. It is probable that the heavily stained granules were 

 microspores, and development from these accounted for the uniformly small cells ob- 

 served in this area. It is probable that the mucilaginous habit is adopted as protection 

 against the severe climatic conditions which prevail so far south, and owing to the short- 

 ness of the season, the production of microspores would be the only method that would 

 enable the flora to reproduce effectively and maintain the standard of productivity that 

 is so constant a character in polar waters. In the Ross Sea, Corethron criophilum existed 

 in a pure state but was sometimes found associated with several species of Chaetoceros. 



D XVI 2j 



