The following key was published by Bobrov and Mazei (2017):
| 1 | Shell is distinctly compressed laterally | 2 | | – | Shell is not compressed or slightly compressed laterally | 7 | | 2 | Oval aperture | C. compressa angustioris | | – | Circular aperture | 3 | | 3 | Shell length 45–63 μm | C. angustastoma | | – | Shell length 32–45 μm | C. paludosa | | — | Shell is smaller than 25 μm | 4 | | 4 | Shell width is higher than length | C. compressa australis | | – | Shell width is smaller than length | 5 | | 5 | Shell lanceolate | C. lanceolata | | – | Shell pyriform | C. lanceolata | | – | Shell ovate | 6 | | 6 | There is a thick organic lip around an aperture | C. compressa | | – | There is no lip around an aperture | C. compressa ovata | | 7 | Shell surface is crenulate | 8 | | – | Shell surface is not crenulate | 10 | | 8 | Shell shape is ovoid | 9 | | – | Shell shape is globose or subglobose | C. crenulata globosa | | 9 | Shell length is lower than 20 μm | C. crenulata | | – | Shell length is higher than 23 μm | C. splendida | | 10 | Shell is pyriform with prominent neck; surface covered by rare sand grains | 11 | | – | Shell is ovoid or globular, without prominent neck | 13 | | 11 | Shell surface covered by rare sand grains | 12 | | – | Shell surface smooth | C. collum | | 12 | Shell length 16–26 μm; aperture 4–5 μm; neck length is 1/3 of shell length | C. sacculus | | – | Shell length 27–30 μm; aperture 10–13 μm; neck length is 1/4 of shell length | C. sacculus sakotschawi | | 13 | Lateral sides of the shell are conic (angular) | C. angulata | | – | Lateral sides of the shell are rounded | 14 | | 14 | Aperture diameter is equal to shell width | 15 | | – | Aperture diameter is considerably smaller than shell width | 16 | | 15 | Shell width is equal to shell length | C. pusilla | | – | Shell width is considerably smaller than shell length | C. pusilla conica | | 16 | Shell length is higher than 40 μm | 27 | | – | Shell length is lower than 40 μm | 17 | | 17 | Shell length 8–12 μm | 18 | | – | Shell length is higher | 20 | | 18 | Shell length is higher than shell width | C. minuta | | – | Shell length is considerably smaller than shell width | 19 | | 19 | Shell surface is covered by scarce sand grains | C. patinata | | – | Shell surface is smooth | C. bassini | | 20 | Shell surface is covered by scarce sand grains | 25 | | – | Shell surface is smooth | 21 | | 21 | Aperture large, its diameter is about 1/3–1/4 of maximal shell width | 23 | | – | Aperture small, its diameter is about 1/6–1/8 of maximal shell width | 22 | | 22 | Aperture diameter 1.5–2.5 μm | C. leachi | | – | Aperture diameter 3.5–4.5 μm | C. vulgaris | | 23 | Shell ovoid | 24 | | – | Shell pyriform | C. crenulata glabra | | 24 | Shell colorless | C. oviformis | | – | Shell yellowish-brown, oval | C. oviformis fusca | | 25 | Shell subsphaerical, width/length ratio is higher than 0.8 | C. horrida | | – | Shell elongated, width/length ratio is lower than 0.7 | 26 | | 26 | Shell length 24–27 μm | C. voigti | | – | Shell length 20–24 μm | C. psammophila | | — | Shell length 17–21 μm | C. angusta | | 27 | Shell length 40 μm, broadest width at the middle of the shell | C. apiculata | | – | Shell length 55 μm, broadest width at the upper third of the shell | C. valida | | | |
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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
Shell about 55 µm long; ovoid; without neck
C. valida
–
Shell 45-63 µm long; ovoid or circular; with very short neck
–
Shell 30 µm or less in length
3
2
Shell strongly compressed in transverse section
3
–
Shell circular or only slightly compressed
6
3
Aperture oval; test 19-20 µm long
C. compressa f. angustioris
4
Shell broader than long; 16-19 µm long
C. compressa australis
–
Shell longer than broad, or length and breadth equal
5
5
Breadth approaching length; often with collarette (thickened rim); 16-20 µm long
–
Shell narrower, more ovoid; without collarette; 19 µm long
C. compressa ovata
7
Shell ovoid; 17-20 µm long
–
Shell subglobose; 17-20 µm long
C. crenulata globosa
8
Shell flask-shaped, narrowed to a neck above aperture
9
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
Shell as broad as long; sides almost vertical; 10 µm long
–
Shell longer than broad; 12 µm long
14
Length 10-13 µm; rim of aperture 2-3 µm
15
Elongate; often with adherent particles; 24-27 µm long
C. voigti
16
Shell colorless, yellowish, or brownish; encysted form with rounded protoplast and protective diaphragm inside test; 15-23 µm long
–
Shell 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
References:
Page, F. C. (1966). Cryptodifflugia operculata n. sp. (Rhizopodea: Arcellinida, Cryptodifflugiidae) and the status of the genus Cryptodifflugia – Trans. Amer. Microsc. Soc. 85(4): 506-515
An updated key is given by:
Bobrov, A. and Mazei, Y. (2017), A review of testate amoeba genus Cryptodifflugia Penard, 1890 (Phryganellina: Cryptodifflugiidae) with a key to species. Zootaxa 4282 (2): 292–308