Reference : Biol. Bull., 149 : 287—315. (October, 1975) BIOTIC CENSUS OF CAPE COD BAY : DALE R CALDER Marine Resources Research Institute, P. 0. Box 12559, Charleston, South Carolina 29412 The Biotic Census of Cape Cod Bay was initiated in 1965 by the Systematics Ecology Program at the Marine Biological Laboratory, Woods Hole, Massachusetts. Completed in 1969, the census involved qualitative and quantitative sampling of the benthos to determine the species occurring in the bay, and to provide informa tion about their ecology. This report is an account of the hydroids collected during the project. The hydroids of the northeast have received greater attention than those from any other geographic area of North America. Over 75 papers bearing on the taxonomy and distribution of hydroids from New England and Atlantic Canada have been published since Stimpson's (1854) synopsis of the marine invertebrates of Grand Manan Island. Among those not already cited in Fraser's (1944) monograph are papers by Ruebush (1939), Crowell (1945, 1947), Fraser (1945), Berrill (1948), Merrill (1967), Bush and Zinn (1970), Templeman (1973), and Calder (1974), in which one or two species have been discussed. Others, including Préfontaine (1932), Prat (1933), Procter (1933), Dexter (1944, 1947), Fuller (1946), Woods Hole Oceanographic Institution (1952), Bousfield and Leim (1960), Brunel (1962, 1963, 1970), Pr@fontaine and Brunel (1962), Caddy (1970), and Bourget and Lacroix (1973), have included hydroids in a general biotic survey or in a study on fouling organisms. Vervoort (1972) included several species of hydroids from the northeast collected during cruises from the Lamont-Doherty Geological Observatory. A number of hydroid species from Newfoundland and Anticosti Island have been described by Leloup (1939, 1960), and papers on the hydroids of northern Canada by Calder (1970, 1972) included a number of species from the Strait of Belle Isle. Further sources of information on hydroids of the northeast include Fraser's (1946) book on distribution and rela tionship in American hydroids, and the identification manuals of Miner (1950), Smith (1964), and Gosner (1971). Despite this volume of work, little specific information is available concerning the hydroids of Cape Cod Bay. The collection of hydroids from the biotic census was relatively small; Cape Cod Bay apparently does not have an especially rich hydroid fauna. Nevertheless, a number of interesting hydroids were present in the collection, particularly those species adapted to substrates of mud and sand. The systematic arrangement used in this report differs significantly from that employed by Fraser (1944). While Fraser's contributions to North American hydroid taxonomy and zoogeography remain of monumental value, the system of classification in his 1944 monograph is little advanced over that used 75 years earlier. Unfortunately, no comprehensive treatise on the hydroids of North 1 Contribution No. 44 from tbe South Carolina Marine Resources Center, Charleston, South Carolina 29412. 287