The Middle Cambrian ptychoparioids, the only group of trilobites from the limestone conglomerates of the lower St. Lawrence valley of Quebec not treated in the author's previous papers, are described. New genera proposed, with type species in parenthesis, are: Catillicephalites (Catillicephalites marginatus, n. sp.), Kingstonioides (Kingstonioides laevigatus, n. sp), Metisella (Metisella microphthalma, n. sp.), Spencella (Spencella montanensis, n. sp.). 26 new species are discussed. Three unnamed trilobites are also discussed and illustrated.