The following key was published by Page (1966) who noticed: "Professor Tarnogradsky and the fine distinctions made by Playfair on the basis of test form, the following key is of a provisional nature but should be of value especially for identification of more clearly marked species."


  1 Test about 55 µm long; ovoid; without neck C. valida
  - Test 45-63 µm long; ovoid or circular; with very short neck C. angustastoma
  - Test 30 µm or less in length 2
  2 Test strongly compressed in transverse section 3
  - Test circular or only slightly compressed 6
  3 Aperture oval; test 19-20 µm long C. compressa f. angustioris
  - Aperture circular 4
  4 Test broader than long; 16-19 µm long C. compressa australis 
  - Test longer than broad, or length and breadth equal  5
  5 Breadth approaching length; often with collarette (thickened rim); 16-20 µm long C. compressa
  - Test narrower, more ovoid; without collarette; 19 µm long C. compressa ovata 
  6 Test crenulate 7
  - Test not crenulate 8
  7 Test ovoid; 17-20 µm long C. crenulata 
  - Test subglobose; 17-20 µm long C. crenulata globosa
  8 Test flask-shaped, narrowed to a neck above aperture 9
  - Test without neck 11
  9 No adherent foreign particles; brownish; sometimes with two necks and apertures; 14-18 µm long C. turfacea 
  - With adherent foreign particles 10
10 With adherent particles and frequently diatoms; protoplasm not filling much more than half the test; test 17-26 µm long C. sacculus 
  - Diatoms lacking among adherent particles; neck shorter than in above; protoplasm almost filling test; 27-30 µm long C. sacculus sakotschatwi
11 Sides of test rather angular in longitudinal view; 13-14 µm long C. angulata 
  - Sides of test rounded, not angular 12
12 Aperture and oral end of test nearly as broad as greatest diameter of test 13
  - Aperture and oral end of test much narrower than greatest breadth 14
13 Test as broad as long; sides almost vertical; 10 µm long C. pusilla
  - Test longer than broad; 12 µm long C. pusilla conica 
14 Length 10-13 µm; rim of aperture 2-3 µm C. minuta 
  - Larger 15
15 Elongate; often with adherent particles; 24-27 µm long C. voigti 
  - Oval; usually smaller 16
16 Test colorless, yellowish, or brownish; encysted form with rounded protoplast and protective diaphragm inside test; 15-23 µm long C. oviformis 
  - Test colorless or yellowish; encysted form with protoplast filling test and with operculum in aperture; 15-25 µm long C. operculata 


Below authorities are given to facilitate location of specific descriptions:

C. valida Playfair, 1917
C. compressa Penard, 1902
C. compressa ovata Playfair, 1917
C. compressa f. angustioris Tarnogradsky, 1959
C. compressa australis Playfair, 1917
C. sacculus Penard, 1902
C. sacculus sakotschatwi Tamogradsky, 1959
C. angulata Playfair, 1917
C. crenulata Playfair, 1917
C. crenulata globosa Playfair, 1917
C. operculata Page, 1966
C. oviformis Penard, 1890
C. voigti Schmidt, 1926
C. minuta Playfair, 1917
C. pusilla conica Playfair, 1917
C. pusilla Playfair, 1917
C. lanceolata, Golemansky, 1970

Literature:Page, F. C.: Cryptodifflugia operculata n. sp. (Rhizopodea: Arcellinida, Cryptodifflugiidae) and the status of the genus Cryptodifflugia - Trans. Amer. Microsc. Soc. 85(4): 506-515. 1966