Species [var. contractum]: Cell body 28-41 μm long, 18-31 μm wide, isthmus 5.3-8.7 μm wide, L/W=1.5; semicells broad ellipsoid, laterally circular; cellwall smooth or finely dotted, a single pyrenoid in each semicell.
[var. ellipsoideum (Elfving) W. et G.S. West]: Cell body 31-51 μm long, 24-42 μm wide, isthmus 7-12.5 μm wide, L/W=1.3; apex of semicells slightly flattened; cellwall smooth.
[f. jacobsenii (Roy) W. et G.S. West]: Semicells exactly ellipsoid, 28-47.6 μm long, 18-36.4 μm wide, isthmus 4.5-11.3 μm wide.
[var. minutum (Delponte) W. et G.S. West]: Cell body 19-28 μm long, 14-22 μm wide, isthmus 4.3-5.6 μm wide; semicells elongated ellipsoid, laterally circular, cellwall dotted.
(Illustrations of The Japanese Fresh-water Algae, 1977).

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var. contractum: Semicells broad ellipsoid, 28-41 x 18-31 μm
C. contractum v. contractum 41.5 x 32 μm L/W=1.3 Yukawa-numa Nishiwaga Iwate, 2007	C. contractum v. contractum 39 x 32 μm L/W=1.2 Iroha-numa Zao mountain Yamagata, 2005	C. contractum v. contractum 39-40 x 29-32 μm L/W=1.2-1.4 Iroha-numa 1 Zao mountain Yamagata, 2006	C. contractum v. contractum 28 x 23.5 μm L/W=1.2 Iroha-numa 3 Zao mountain Yamagata, 2006
C. contractum v. contractum 48 x 37 μm L/W= Miyatoko m.3 Minami-Aizu Fukushima, 2008	C. contractum v. contractum 32-39 x 25-28 μm L/W= Kashibaru marsh 5 Karatsu Saga, 2006	stock Kashibaru-1 v. contractum 44 x 31 μm L/W= Kashibaru marsh 1 Karatsu Saga, 2006	C. contractum v. contractum μm L/W= Kashibaru marsh Karatsu Saga, 2008
C. contractum ? v. contractum μm L/W= Kitafukuda m.p. Yuri-Honjo Akita, 2008	C. contractum v. contractum 33.5 x 30 μm L/W=1.1 USA, 1999	C. contractum v. contractum 34 x 27 μm L/W=1.3 USA, 1999	C. contractum v. ellipsoideum μm L/W=1.6 Kareki-numa m. Nikko Tochigi, 2008
var. ellipsoideum: apex of semicells flattened, 31-51 x 24-42 μm
C. contractum v. ellipsoideum 29 x 22 μm L/W=1.3 water reservoir near Yamaguchi Univ. Yamaguchi, 2004	C. contractum v. ellipsoideum μm L/W= Kannon-numa 11 Shimogo Fukushima, 2008	C. contractum v. ellipsoideum 41 x 25 μm L/W=1.6 Kareki-numa m. Nikko Tochigi, 2008
f. jacobsenii: Semicells exactly ellipsoid, 28-47.6 x 18-36.4 μm
C. contractum f. jacobsenii 44-46 x 29-31 μm L/W=1.5 water reservoir near Yamaguchi Univ. Yamaguchi, 2004	C. contractum f. jacobsenii 39 x 29 μm L/W=1.5 Koigakubo marsh Tessei-cho Okayama, 2004	C. contractum f. jacobsenii μm L/W= Kareki-numa m. Nikko Tochigi, 2008	C. contractum ? or C. depressum ? 29 x 24 μm L/W=1.2 Uruguay, 1999
var. minutum: Semicells elongated ellipsoid, 19-28 x 14-22 μm
C. contractum v. minutum 21 x 15 μm L/W=1.4 Omine-numa Minakami Gunma, 2006	C. contractum v. minutum 24 x 16 μm L/W=1.5 Omine-numa Minakami Gunma, 2007.4	C. contractum v. minutum 25 x 16 μm L/W=1.6 Omine mt. Minakami Gunma, 2007.4	C. contractum v. minutum μm L/W= Omine-numa Minakami Gunma, 2007.10
C. contractum v. minutum μm near Omine-numa Minakami Gunma, 2008	C. contractum v. minutum ? 25 x 19 μm L/W=1.3 Kumakubo Nasu-shiobara Tochigi, Aug. 05	C. contractum v. minutum ? 24.5 x 19 μm L/W=1.3 Kumakubo Nasu-shiobara Tochigi, Dec. 05	C. contractum v. minutum 19 x 15 μm L/W=1.3 Nanbata castle p. Fujimi Saitama, 2005
C. contractum v. minutum 20.5 x 16 μm L/W=1.3 Tane-ike Togakushi highland Nagano, 2005	C. contractum v. minutum 16 x 13 μm L/W=1.2 Azami-ike Norikura highland Nagano, 2006	C. contractum v. minutum ? 18 x 15 μm L/W=1.2 Maimeno-ike Norikura highland Nagano, 2006	C. contractum v. minutum ? 25.5 x 20 μm L/W=1.3 Kodanuki marsh Fujinomiya Shizuoka, 2006

Similar species -- >> C. depressum; C. bioculatum; C. moniliforme;

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