ASTRORHIZIDAE 69 



Test monothalamous, pear-shaped with a well-defined aperture at the narrow end, 

 aboral end rounded. Wall thin, very smooth but unpohshed, constructed of very minute 

 particles without visible cement. Colour light grey. Size about 0-5 mm. long, 0-25 mm. 

 broad. 



Frequent at WS 28 and 32, ver}^ rare at the other stations. The shape of this little 

 organism agrees with that of Proteo?ima diffli/gifor?nis, but the character of the test is 

 quite distinctive and indicative of relationship to Hippocrepina oviformis, Heron-Allen 

 and Earland. In life, the test is probably flexible, several of our specimens being 

 collapsed without fracture. There can be little doubt that the South Georgia specimens 

 are identical with Technitella flexibilis, Wiesner. He describes his species as snow-white, 

 oval, very flexible when wet but collapsing when dried. The dry shell is said to be rigid 

 and comparatively strong, built of the finest possible mineral fragments. The aperture is 

 small and circular on a produced neck. Wiesner's figures agree with the South Georgian 

 specimens, but I do not agree with his attribution of the species to Technitella, the shell 

 structure being quite different and showing no selective tendency. 



Genus Hippocrepinella, Heron-Allen and Earland, 1932 



Test free, monothalamous, irregularly cylindrical and sometimes curved, furnished 

 with two terminal apertures. Wall thin compared with the size of the large central 

 cavity, constructed of extremely fine sand and mud with little cement, and generally 

 without inclusion of larger particles ; smoothly and neatly finished, but often exhibiting 

 numerous fine transverse wrinkles. It is probably flexible during life, but dry specimens 

 are rigid and fragile. Colour varying from white to very dark grey. 



Although widely distributed round the coast-line of South Georgia, Hippocrepinella 

 is mainly characteristic of the Cumberland Bay area, the majority of the records being 

 from stations in or near that bay. It favours the tenacious mud found in that area, 

 although a few specimens have been recorded elsewhere on sandy bottoms. 



Hippocrepinella appears to be closely related to Hippocrepina. Indeed, but for the 

 existence of the secondary aperture, we should have had no hesitation in referring the 

 specimens to that genus, as the wall of the test is very similar in character though more 

 delicate. Also, owing to the finer materials employed in its construction, the surface of 

 the test is smoother and more polished. 



Of the two apertures, one, which may be regarded as the principal oral opening, is 

 always well defined and sometimes quite large, while the secondary or basal opening is 

 usually inconspicuous, and sometimes only to be detected with difficulty. 



There is little doubt that the test is flexible and extensible in life. The apertural ends 

 probably expand for the absorption of food and contract for digestion, opening again 

 for the rejection of the empty Diatom shells, which form the food of the animal. Diatom 

 valves have been observed inside the cavity, of dimensions larger than the aperture. The 

 flexibility of the living test would also account for the curvature of some specimens and 

 the transverse wrinkles observed in others. 



6-z 



