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.”

Key to the species
1
Test about 55 µm long; ovoid; without neck
C. valida
Test 45-63 µm long; ovoid or circular; with very short neck
Test 30 µm or less in length
3
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
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
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
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
Test longer than broad; 12 µm long
14
Length 10-13 µm; rim of aperture 2-3 µm
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
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

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