Vexillum exostium is a species of small sea snail, marine gastropod mollusk in the family Costellariidae, the ribbed miters. Description The length of the shell attains 24.8 mm. Distribution This marine species was found off the Dampier Archipelago, Western Australia. References Marrow M.P. (2019). Seven new species of Vexillum (Gastropoda: Costellariidae) from Western Australia. Acta Conchyliorum. 19: 77–94. exostium Gastropods described in 2019 Gastropods of Australia