A new species of Proteocephalus, P. joanae sp. n„ is described from the intestine of the colubrid snake, Xenodon neuwiedi (Günther, 1863) (Serpentes: Xenodontinae), from Brazil. The new species differs from all other members of Proteocephalus by possessing a swollen elongated posterior part of the scolex herein considered as a metascolex. Furthermore, it is the only member of New World Proteocephalus possessing a voluminous glandular apical organ larger than suckers. P. joanae is also characterized by very elongated gravid proglottides. This is the first member of the Proteocephalidea occurring in Xenodon. Even though the species differs significantly from other species of the genus Proteocephalus, which currently contains many species that are morphologically very distinctive, it seems prudent to refrain from reworking the classification of the group until accurate redescriptions of Neotropical species can be conducted, preferably based on examination of type and freshly collected material. The definition of the metascolex is discussed herein.