GENERAL SYSTEMATIC BACTERIOLOGY 85 



Key to the Families of Thiobacteriales 



A. Cells containing sulphur granules (or in one species possibly oxalate crystals), 



but no bacteriopurpurin. 



1. Unicellular, motile forms. Not filamentous. 



Family I. Achromatiaceae 



2. Filamentous forms Family II. Beggiatoaceae 



B. Cells containing bacteriopurpurin with or without sulphur granules. 



Family III. Rhodobacteriaceae 



Key to the Genera of Achromatiaceae 



A. Cells spherical or ellipsoidal. 



1. Cells ellipsoidal (spherical when newly divided). Cells containing gran- 



ules of calcium oxalate (perhaps sulfur). 



Genus 1. Achromatium 



2. Cells spherical, with sulphur granules in a central vacuole. 



Genus 2. Thiophysa 



B. Cells longer, very large (42 to 86ju) with peritrichous flagella. 



Genus 3. Hillhousia 



Key to the Genera of Beggiatoaceae 



A. Filament non-motile, with a contrast to base and tip, attached. 



Genus 1. Thiothrix 



B. Filaments motile (oscillating) not attached, no differentiation into tip and 



base. 



1. Filaments not in bundles nor surrounded by a gelatinous sheath. 



Genus 2. Beggiatoa 



2. Filaments in bundles, surrounded by a gelatinous sheath. 



Genus 3. Thioploca 



Key to the Subfamilies of Rhodobacteriaceae 



A. Cells containing sulphur granules. 



Subfamily I. Chromatioideae 



B. Cells without sulphur granules.. .Subfamily II. Rhodobacterioideae 



Key to the Tribes of Chromatioideae 



A. Cells united, at least during a part of the life history, into families. 



I. Cell division such that masses of cells, not merely plates, are formed. 



a. Cell division in three directions of space. 



Tribe I. Thiocapseae 



b. Cell division first in three, then in two directions of space. 



Tribe II. Lamprocysteae 

 II. Cell division in two planes, forming plates of cells. 



Tribe III. Thiopedieae 

 III. Cell division in one plane. .Tribe IV. Amoebobacterieae 



B. Cells free, capable of swarming at any time 



Tribe V. Chromatieae 



