A new species of Falcidens (Mollusca, Aplacophora, Caudofoveata) from the southeastern Brazilian coast: external anatomy, distribution, and comparison with Falcidens caudatus (Heath, 1918) from the USA

Falcidens australocaudatus sp. nov. is described from the southeastern Brazilian coast, based on characters of external anatomy, including size, shape, and ornamentation of the sclerites, and on the structure of the radula. It bears a tail characteristically distinct from the trunk, a U-shaped oral shield, and the sclerites of the trunk and shank are slender, without an obvious separation between a blade and base; it differs from all other tailed species of Falcidens, which have sclerites that are flared at the base. Externally, it is very similar to F. caudatus, recorded from the northeastern coast of the USA; the sclerites of this species are illustrated here and are characterized as being larger, slightly flared, and with a different ornamentation as compared to those of the new Brazilian species. Based on the examination of a large collection, the new species can be characterized as very abundant at the southeastern Brazilian coast, with a wide bathymetric distribution, but mainly occurring in the upper continental slope.