462 R. E. BUCHANAN 



The order Thiobacteriales may be divided into families as 

 follows : 



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. AchromatiacecB 



2. Filamentous forms Family II. Beggiatoaceae 



B. Cells containing bacteriopurpurin with or without sulphur granules. 



Family III. Rhodobacteriaceo' 



Family I. Achromatiaceae Fam. nov. 



Unicellular, large, motile {by means of flagellar) cells containing 

 granules of sulphur {or in one form possibly oxalate), but no bac- 

 teriopurpurin. 



The following key will separate the genera recognized. 



Key to the genera of Achromatiaceae 



A. Cells spherical or ellipsoidal 



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



granules of calcium oxalate (perhaps sulfur). 



Genus I. Achromatium 



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



Genus 2. Thiophysa 



B. Cells longer, very large (42 to 86ai) with peritrichous fiagella. 



Genus. 3. Hillhousia 



Genus 1. Achromatium Schewiakoff 1893 



Synonyms : 



Modderula Frenzel, 1897, p. 901 



Cells large, nearly spherical in newly divided cells to ellipsoidal, 

 15 to 4-3 by 9 to 22 ix. Cells closely packed with large granules, 

 ai first interpreted as sulphur, but later interpreted as calcium oxalate. 

 When granules are dissolved, cells show coarse alveolar structure. 

 Cells are motile, fiagella not demonstrated. Cell division resembles 

 the constriction of flagellates rather than the fission characteristic of 

 bacteria. 



The type species is Achromatiurn oxaliferum Schewiakoff. 

 The organism occurs in the slime at the bottom of the rivers, in 

 the so called "Modder." 



