THE APLACOPHORAN FAMILY PROCHAETODERMATIDAE IN THE NORTH AMERICAN BASIN, INCLUDING CHEVRODERMA N.G. AND SPATHODERMA N.G. (MOLLUSCA; CHAETODERMOMORPHA).

Six species in three genera of Prochaetodermatidae are described from over 650 stations and 5200 specimens in the Atlantic and north Pacific Oceans from depths between 500 and 7300 m. Included are all species in the North American Basin and all species in Chevroderma n.g. Three principal characters differentiate prochaetodermatid Species and genera: spicules, radula, and body shape. Family membership is defined by radula and jaws, spicule morphology determines genus, and species are described by spicules and radula. Mean body shape describes populations of species. Interference colors produced by the aragonite spicules indicate spicule thickness and symmetry. The variation in Prochaetoderma yongei n. sp., described in detail, establishes the taxonomic base on which to judge the morphological limits of a prochaetodermatid species. Spaihoderma n.g. and Chevroderma n.g. differ from each other and from the genus Prochaetoderma in spicule morphology. P. yongei and S. clenchi n. sp. are widespread northwestern and eastern Atlantic continental slope and abyssal rise species. C. turnerae and C. gauson n. spp. are abyssal species, the former occurring throughout the Atlantic, the latter only in the northern West European Basin. C. scalpellum n. sp. is a slope species of restricted range in the eastern Atlantic. C. whitlatchi n. sp. is a wide-ranging abyssal and hadal species of the northern east and mid-Pacific. A wide geographic range is correlated with a vertical depth distribution greater than 1500 m. All species are patchy in distribution but particular species can be numerically dominant and occur at high densities locally, e.g., up to 400 m for P. yongei and 178 m for C. whitlatchi. In the north Atlantic, greatest numerical abundances and lowest diversity of Prochaetodermatidae occur in the North American Basin.