Abstract
On the basis of type material and new collections of NW Atlantic marine Tubificidae, the definition of Smithsonidrilus Brinkhurst, 1966 is considerably revised, and its only known species, S. marinus Brinkhurst, 1966, is redescribed. S. marinus is characterized by very complex male efferent ducts, the morphology of which was previously only fragmentarily known. The author distinguishes between S. marinus marinus, known from western and south-eastern parts of Florida, and S. marinus westoni subsp. n., which is recorded from off the eastern US coast (New Jersey through N Carolina), the Bahamas, and Bermuda. The two subspecies differ from each other principally in the morphology of their spermathecae.