Genus I. Achromatium Schewiakqff, 1893. (Schewiakoff, tjber einen neuen bacterienahnlichen Organismus des Siisswassers, Habilitationsschrift, Heidelberg, 1893; Hillhousia West and Griffiths, Proc. Roy. Soc, B, 81, 1909, 389.) A.chro.ma'ti.um. Gr. pref. a not; Gr. noun chromatium color, paint; M.L. neut.n. Achro- matium (that which is) not colored. Description as for the family. It is not easy as yet to determine whether several species should be recognized in this genus. There appears to be some justification for differentiating between the forms which contain the characteristic and conspicuous calcium carbonate inclusions and those in which these large spherules are lacking. The former have been reported mostly from fresh- or brackish-water environments, while the characteristic habitat of the latter seems to be marine. It is, of course, probable that the internal deposition of calcium carbonate depends upon the composition of the environment, so that the distinction may prove arbitrary and non-specific. Achromatium cells of widely different sizes have been described. Schewiakoff (op. cit., 1893) mentions a variation of 9 to 22 microns in width and of 15 to 43 microns in length for Achromatium. oxaliferwn. Larger cells have been observed by Warming (Videnskab. Meddel. naturhistor. Foren., Kjobenhavn, 1875, No. 20-28, 360; size to 85 microns) and by Virieux (Ann. Sci. Natur., Ser. 9, 18, 1913, 265; size to 95 microns in length). Nadson (Bull. Jard. Imp. Botan., St. Petersb., 13, 1913, 106; also see Jour. Microb., St. Petersb., 1, 1914, 52) proposed the name Achromatium gigas for the larger organisms; also West and Griffiths (Ann. Bot., 27, 1913, 83) created two species: Hillhousia mirabilis, with sizes of 20 to 33 by 42 to 86 microns, and Hillhousia palustris, measuring, on the average, 14 by 25 microns, for the same group of sulfur bacteria. However, Bersa (Sitzungsber. Akad. Wiss., Wien, Math- em-naturw. Kl., I, 129, 1920, 233) observed so manj^ intermediate sizes that he recognized only a single species. Nadson and Wislouch (Bull. Princ. Jard. Botan., Republ. Russe, 22, 1923, Suppl. 1, 33) arrived at the same conclusion, and this view is accepted here. The marine Achromatium types, which do not contain calcium carbonate crystals, have also been segregated into species on the basis of their size. Here again there does not seem to be any valid reason for maintaining several species as there is a continuous series of in- termediate forms. Thus the organisms previouslj^ described as Achromatium oxaliferum., Achromatium gigas, Hillhousia mirabilis and Hillhousia palustris are provisionally treated here as one species, while the marine counterpart, Thiophysa voluians, is combined with Thiophysa macrophysa and Thiosphaerella amylifera, all three being regarded as Achromatium volutans. The type species is Achromatium oxaliferum Schewiakoff. Key to the species of genus Achromatium. I. Characteristically contains calcium carbonate crystals in the form of highly refractile, large spherules; occurs mostly in fresh-water and brackish muds. 1. Achromatium oxaliferum. II. Occurs naturally without such calcium carbonate inclusions in marine mud. 2. Achromatium volutans. 1. Achromatium oxaliferum Schewia- son, Bull. Jard. Imp. Bot., St. Peter.sb., koff, 1893. (Schewiakoff, Uber einen neuen 13, 1913, 106.) bacterienahnlichen Organismus des Suss- o.xa.li'fe.rum. Gr. noun oxalis sorrel, a wassers, Habilitationsschrift, Heidelberg, sour plant; M.L. noun oxalatum oxalate; 1893; Hillhousia mirabilis West and Grif- L. v.fero to carry; M.L. adj. oxaliferus ox- fiths, Proc. Roy. Soc, B, 81, 1909, 389; Hill- alate-containing. housia palustris West and Griffiths, Ann. Unicellular organisms varying in shape Bot., 27, 1913, 83; Achromatium gigas Nad- from spherical or ovoid to shortly cylindri-
