A unique pedunculate barnacle, Arcoscalpellum spinum, new species, was discovered on the East Pacific Rise (18°S) during the 1993 "Naudur" cruise of the French submersible Nautile. We propose Vertebroscalpellum, new subgenus, to include this and a related species, A. brevecarinatum(Hoek), from the south Atlantic. Although basically 8 plates comprise each whorl of the peduncular armament in scalpellomorphs, the number in derived forms is subject to variation. The number of vertical tiers of peduncular plates generally relates to the number of capitular plates and/or the sutures between them. One trend is toward the multiplication of plates, the other a reduction. The latter is manifest in Vertebroscalpellun by the separation of 5-plated whorls into an imbricating 2/3 plated pattern that is unique among scalpellomorphs.
