Polychaetous annelids from deep water off western Mexico were first collected by the ALBATROSS expeditions of 1886-91 and 1911. The material from these expeditions was treated by Chamberlin (1919) and Treadwell (1923). Chamberlin named a large number of new species, most of which have not been recovered from other areas and indicate the presence of a deep-water fauna characteristic of the eastern Pacific Ocean.
The Allan Hancock Foundation collected benthic organisms off western Mexico from 1932 to 1942 and during a series of cruises since 1948. These collections have been treated by Hartman and Fauchald in a number of papers. The Scripps Institution of Oceanography has had a series of cruises, especially in the Gulf of California, where benthic samples were taken. Parts of this material were treated by Parker (1964a, 1964b) in papers dealing with the benthic ecology of the gulf and adjacent areas. Parker's study was based mainly on the mollusks; the polychaetous annelids are treated here for the first time.
Two cruises with the VELERO IV of the Allan Hancock Foundation were undertaken in 1967 and 1970 to investigate further the benthic fauna off western Mexico. The material from these two expeditions forms the basis of this study, but other materials have been included as indicated below.
Forty-nine polychaetes in fourteen families were previously known from deep water off western Mexico. These are listed in Table 1 with reference to the records for each species. The present collection includes 227 species in forty-three families. Seventy-six new species are named and revisions are made of some taxa, where materials and previous investigations permitted complete surveys.