Suborder: Ventral cirri not as distinct files, except Gastrostyla.
Family: Frontoventral cirri scattered on forward, mid- and rear areas of ventrum.
Genus: Marginal cirri not confluent at rear; oral cavity wide, expansive; caudal cirri (usually 3) prominent (Illustrated Guide, 1985). Cell body ovoid to elongate, inflexible; large AZM supported anteriorly by a collar; two rows of marginal cirri not confluent posteriorly; strongly-developed frontoventral & transverse cirri present; three long, stiff caudal cilia; dorsal surface ornamented by short rows of cirri; two macronuclei; cyst stage recorded in some species (Carey, 1992).
8 (3 + 2 + 3) frontal, 5 ventral, 5 transverse, 3 caudal cirri.
Species: 100-300 μm long (Kudo, 1966). Elongate, anterior wider than posterior; up to 300 μm long; AZM 1/3 to 1/2 body length; distinctive 3 frontal and 3 long caudal cirri; two ovoid macronuclei (Carey, 1992).

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S. mytilus 225 μm posterior rounded Saitama, 1999	S. mytilus 175-200 x 88 μm posterior truncated Japan, 1998	S. mytilus 163 μm posterior truncated Saitama, 1999	S. mytilus μm posterior truncated Ta river Ishibashi Tochigi, 2004
S. mytilus 160 x 65 μm Akebono marsh Tsuchiyu-onsen Fukushima, 2005	S. mytilus 97 μm posterior rounded Saitama, 1999	S. mytilus 144-200 μm posterior pointed Japan, 1998	S. mytilus 153 x 63 μm posterior pointed Japan, 1998
S. mytilus μm posterior pointed USA, 1998	S. mytilus 110 x 46 μm Kiyasse-Hanyu Hanyu Saitama, 2006	S. mytilus ? 119 x 44 μm posterior rounded lateral short cirri Japan, 1998	S. mytilus 145 x 55 μm Koyama River Honjo Saitama, 2006

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