thrix. Found in both fresh-water and marine environments containing hydrogen sulfide or soluble sulfides. In Beggiafoa the species so far recognized have been differentiated on the basis of the diameters of the trichomes. The range of sizes for the several species appears, in most cases, to be quite arbitrary, especially in view of the existence of practically all intermediate di- ameters. In previous editions of the Manual, certain fresh-water forms were sometimes regarded as distinct from the salt-water forms of the same size. However, as their habitats have been found to be not mutually exclusive (Bavendamm, Die farblosen und roten Schwe- felbakterien, Pflanzenforschung, Heft 2, 1924, 104), these organisms are now regarded as a single species irrespective of their habitat. Pure-culture studies may establish more satis- factory methods of differentiation of the species of this genus. The type species is Beggialoa alba (Vaucher) Trevisan. Ketj to the species of genus Beggiatoa. I. Diameter of trichomes is less than 15 microns. A. Diameter of trichomes is greater than 2.5 microns. 1. Diameter of trichomes is less than 5 microns. 1. Beggiatoa alba. 2. Diameter of trichomes is greater than 5 microns. 2. Beggiatoa arachnoidea. B. Diameter of trichomes is less than 2.5 microns. 1. Diameter of trichomes is greater than 1 micron. 3. Beggiatoa leptomitiformis. 2. Diameter of trichomes is less than 1 micron. 4. Beggiatoa minima. II. Diameter of trichomes is greater than 15 microns. A. Diameter of trichomes is greater than 25 microns. 5. Beggiatoa gigantea. B. Diameter of trichomes is less than 25 microns. 6. Beggiatoa mirabilis. 1. Beggiatoa alba (Vaucher, 1803) Tre- 1827) Rabenhorst, 1865. {Oscillatoria arach- visan, 1845. (Oscillatoria alba Vaucher, His- noidea Agardh, Regensburger Flora, 1827, toire des Conferves d'eau douce, 1803, 198; 634; Rabenhorst, Flora europaea algarum, Beggiatoa punctata Trevisan, Prospetto della 1865, 94.) Flora Euganea, 1842, 56; Trevisan, Nomen- a.rach.no.i'de.a. Gr. adj. arachnoides clator Algarum, 1845, 58.) cobweb-like; M.L. adj . arachnoideus cobweb- al'ba. L. adj. albiis white. like. Trichomes, 2.5 to 5.0 (most commonly Trichomes, 5 to 14 microns in diameter, 3.0) microns in diameter, of uniform width. of uniform width. Segments 5 to 7 microns The segments, 3.0 to 9.0 microns long, are in length; segmentation generally observa- practically square shortly after division; ble only after special staining or removal of segmentation is difficult to detect in tri- sulfur globules. Terminal cells are rounded, chomes containing many sulfur globules. Habitat: Found in both fresh-water and Terminal cells are rounded. marine environments containing hydrogen Habitat: Found in both fresh-water and sulfide, marine environments containing hydrogen sulfide. Ubiquitous. Probably the most 3. Beggiatoa leptomitiformis Trevisan, common of the sulfur bacteria which occur 1842. (Prospetto della Flora Euganea, 1842, in trichomes. 56.) lep.to.mi.ti.for'mis. M.L. noun Lep- 2. Beggiatoa arachnoidea (Agardh, lomitus a genus of bacteria; L. noun forma
