Sewellia is a genus of fish in the family Gastromyzontidae. They are commonly called hillstream loaches (a common name shared with family Balitoridae) and are found in Laos and Vietnam. According to Roberts (1998) a possible defining characteristic of the genus Sewellia is their posterior pelvic valve is highly complex and different from other rheophilic loaches (loaches adapted to fast flowing water), their posterior pelvic valve involves the overlapping of the second to last ventrally exposed rays. Adapted to high velocity streams, Sewellia have depressed body shapes that are laterally expanded, thus increasing their hydrodynamical properties and allowing them to better stay attached to rocks in swift current. Sewellia also have paired fins with a single simple ray and numerous branched rays allowing them to grip rocks in swift current.

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