Tetrachaelasma southwardi Newman & Ross, 1971, a bathylasmatine balanomorph, has been recorded from the Mar del Plata Submarine Canyon (ca. 38 degrees S, off the coast of Argentina), at two stations located at significantly different depths (1950 m and 2934 m). A total of 29 specimens, complete or damaged but with soft parts intact, were collected. This unusually large number of well-preserved specimens allows us to add supplementary descriptions and document intraspecific morphological variations. The differences between T. southwardi and T. tasmanicum Buckeridge, 1999, the second species of this genus, are re-evaluated. A map and an updated list including all the records of the genus Tetrachaelasma Newman & Ross, 1971 are provided. The distribution of the genus Tetrachaelasma in the Southern Ocean is discussed. Furthermore, a single specimen of another bathylasmatine balanomorph, which was assigned to the genus Bathylasma Newman & Ross, 1971, was also obtained at the 1950 m station herein studied. This is the first record of the genus Bathylasma from the South-West Atlantic. This specimen has one T. southwardi attached to it, marking the first time that members of these two genera have been found living together.
