ASTRORHIZIDAE 59 



55. Sorosphaera depressa, Heron-Allen and Earland (Plate V, figs. 20, 21). 

 Sorosphaera depressa, Heron-Allen and Earland, 1929, etc., FSA, 1929, p. 102, pi. i, figs, i, 2. 



Eight stations: 17, 27, 136, 144, 148; WS 33, 113, 365. 



Test attached, light to dark grey in colour according to the constituent particles, 

 consisting of a variable number of irregularly shaped chambers spreading without 

 definite plan over a stone or other surface of attachment, and often following a crack or 

 superficial depression for protection. The walls are thin, firmly and smoothly constructed 

 of fine sand grains and cement, but they are dull and unpolished. The outer surface is 

 rougher than the interior wall of the chambers, which have a chitinous lining. 



Each chamber forms a distinct and separate entity enclosed within its own walls and 

 base. There is no sign of any aperture or means of communication between the chambers. 

 Communication with the external medium can be only through minute interstitial 

 openings in the wall of the test. The openings shown in Plate V, fig. 21, are only inci- 

 dental fractures of the wall. 



When a colony is attached to a stone, as in Plate V, fig. 21, the test forms quite a solid 

 object capable of sustaining considerable stress, but when growing on a flexible base, the 

 chambers are readily separable without fracture of the walls of the test (see Plate V, 

 fig. 20). 



Monothalamous Arenacea, when sessile, are usually more or less semi-globular in 

 plan, but in Sorosphaera depressa the chambers are distinctly polygonal. This in some 

 cases is due to the irregularities of the surface of attachment, but specimens have been 

 observed (see Plate V, fig. 20) which suggest that the organisms first form a quadrate 

 colony, and thereafter spread irregularly in the line of greatest protection or least 

 resistance. 



The dimensions of the chambers vary considerably from 0-3 to o-8 mm. in diameter. 

 A colony may cover a space of 0-5 square centimetre. The thickness of the wall is only 

 about 0-02 mm. 



Always very rare, but some good specimens at St. 27 and WS 113, where it was more 

 frequent than at any other station. Both sessile and free individuals were found, the 

 latter always showing evidence in their flat base of having become detached. 



Genus Psammosphaera, F. E. Schulze, 1875 



56. Psammosphaera fusca, Schulze (F 60). 



Thirty-five stations: 17, 30, 42, 123, 126, 140, 144, 148, 151; 53° 00' S, 34° 22' W; WS 27, 28, 

 31, 32, 33, 40, 42, 51, 61, 63, 154, 334, 336, 348, 349, 351, 353, 365, 373, 429, 521, 522, 523; 

 DrygalskiFjord;MS68. 



Generally distributed and sometimes frequent, notably at WS 32 and 154. Excep- 

 tionally large single specimens up to 3-0 mm. in diameter were found at St. 151, WS 63 

 and 521 in company with normal individuals. There is the usual wide range of variation, 

 but the general average for the area is a rather small and neatly constructed form. At 

 those stations where the bottom deposit contains black sand, e.g. St. 148, WS 28 and 



