[1] Pseudaxine bivaginalis was described based on 15 specimens, from the gills of the yellowtail horse mackerel Trachurus novaezelandiae (Carangidae), collected off New Zealand.

In the same paper, Dillon & Hargis emended the diagnosis of the genus Pseudaxine, to include the presence of (1) a paired vaginae opening near the lateral margins at or near the level of the genital atrium, (2) a genito-intestinal canal short and postovarian or relatively long and paraovarian.

The haptor is asymmetrical, with a laterally directed end (the direction in which the end points, right or left, varies individually, but the internal organs appear to maintain a constant orientation regardless of this variation), and bears 23–34 clamps similar in shape but slightly dissimilar in size, arranged in a single row.

The extreme of the haptor carries an elongated proboscis-like process called "the terminal lappet", bearing 2 pairs of gastrocotyloid anchors.

The digestive organs include an anterior subterminal mouth, a pharynx, an oesophagus and a posterior intestine that bifurcates near the level of the genital atrium in two lateral branches.