Abstract
Dolichostomus leucotaenia gen. et sp. nov. is described from two specimens collected from off northern Vietnam. It is characterized by a long, oval mouth opening visible from the ventral view, a feature unique among the order Myxiniformes. It also possesses a single medial nasal‐sinus papilla and branchial slime pores positioned external to the gill‐aperture series. Moreover, it can be further distinguished by its slender body, a distinct white mid‐dorsal stripe, five pairs of gill apertures arranged in a straight line, and ventral aorta not bifurcated. The character states of the relative position of branchial slime pores and gill‐aperture series and bifurcation of ventral aorta suggest that D. leucotaenia sp. nov. represents a transitional lineage between the subfamilies Rubicundinae and Myxininae.