The alcyonarian fauna of the West Indies is prolific and conspicuous and has been known for many years, with the natural result that a great many more species have been described than actually exist. The deep-water fauna, which received little attention prior to the work of VERRILL, was thoroughly reviewed by DEICHMANN in 1936. The shallow-water and reef fauna was the subject of a series of extensive papers by KUKENTHAL and his collaborators, KUNZE, MOSER, RIESS, BIELSCHOWSKY, and TOEPLITZ, but this ambitious study appears to have been based upon inadequate collections and its usefulness is seriously limited by the number of synonyms and misidentifications that it contains. No comprehensive survey of the fauna exists, and there is no satisfactory guide for the identification of specimens. This paper, which was prepared at the request of Dr. P. WAGENAAR HUMMELINCK, Secretary of the Stichting ‘Natuurwetenschappelijke Studiekring voor Suriname en de Nederlandse Antillen’ (Foundation for Scientific Research in Surinam and the Netherlands Antilles), forms such a guide and at the same time reviews the fauna to the extent permitted by the collections in hand and the literature. With Dr. HUMMELINCK’S collection of West Indian octocorals serving as a nucleus, the pertinent material in the collections of the U.S. National Museum was critically revised and correlated with the literature in order to gain an accurate picture of the known fauna. As a result of this study, it was possible to recognize 75 species of alcyonarians belonging to the orders Telestacea, Alcyonacea, Gorgonacea, and Pennatulacea inhabiting the reefs and shallow waters of the warm western Atlantic. An additional 21 species from deeper water are also included for comparative purposes or because they inhabit the transitional zone just below the region of active reef growth. Seventeen species and a few growth forms are described as new to science. Each species is diagnosed and illustrated with drawings of the details of spiculation and, in the case of new or especially common species, photographs of the colonial form. Taxonomic keys with couplets illustrated for clarity are provided to facilitate the identification of specimens. The species described in this paper are arranged as indicated in the Table of Contents (p. 3—7). A total of 96 species are described from the region including the Bermudas, the southeastern coast of the United States, the Bahamas and Antilles, and the east coast of South America south to the reefs of Brazil. Of these, 52 species occur in the reef habitat proper or closely associated with it, and another 23 species occur in depths of 25 fathoms or less. The orders Telestacea, Alcyonacea, and Pennatulacea are togehter represented by only 13 species within the bathymetric limits set forth, the remaining 83 belonging to the order Gorgonacea. The littoral and reef-dwelling representatives of the last-named order belong for the most part to the two families Plexauridae and Gorgoniidae, which include 35 and 34 species respectively. When the shallow-water alcyonarian fauna is added to the deep-water fauna as reported by DEICHMANN, a total of 196 species is revealed for the area. This is a fauna of only modest proportions when compared with that of the East Indies, where some 445 species (exclusive of Pennatulacea) were obtained by the ‘Siboga’ Expedition, but nevertheless, the gorgonians are the dominant sessile animals on many of the reefs of Florida, the Bahamas, and the Antilles. This dense population consists chiefly of about a dozen species, all the others being rare or of local occurrence, so it appears that the reef fauna is rich in individuals but poor in species. The distribution of alcyonarians is influenced by a variety of factors, among them salinity, temperature, illumination, depth of water, and character of the bottom. It is not possible to single out any one factor as the most important, since they all interact closely, but there is no doubt that temperature is one of the most influential. Although temperature requirements and tolerations have not been determined experimentally for alcyonarians, they can reasonably be assumed to parallel more or less closely those of the principal reef-formers. It has been observed that formation of reefs does not take place in waters that drop below 68°F. for any appreciable period during the winter. Since active growth of reefs occurs at Bermuda, the northernmost limit of the West Indian fauna, its annual minimum temperature of 66°F, may be taken as the limit for reef formation in the West Indian area. Tropical alcyonarians occur up to this minimum isotherm of both coasts of Florida. Most alcyonarians are stenohaline and require salinities within the range found in the open sea. However, the occurrence of a few species, such as Leptogorgia setacea of the southeastern coast of the United States, in the brackish inshore waters of bays and river mouths indicates that a limited degree of euryhalinity does occur in the Octocorallia. A rough and solid bottom is apparently as necessary for the attachment of gorgonian planulae as it is for those of madrepores, and the importance of this requirement is clearly demonstrated on the west coast of Florida, where reef communities gain a foothold only on the scattered solid outcrops on an otherwise broad, sandy shelf. A few species of Gorgonacea are known to live unattached, the colonies apparently doing so in some cases because no suitable objects were available for attachment, in others because they were broken loose from their original solid support but continued to live in a prone position. Certain deep-water gorgonacean groups (families Chrysogorgiidae and Isididae) that inhabit areas with a scarcity of solid material are able to adapt the form of their holdfast to the conditions present at the time of metamorphosis, producing either a calcareous basal disk for attachment to shells and stones, or a branched, rootlike process for anchoring the colony firmly in a muddy bottom. The pennatulaceans, which are adapted for life on soft bottoms, require either sand or mud and therefore are not found closely associated with reef communities. The octocorals of the reefs are restricted bathymetrically to the upper 25 fathoms of water, perhaps because of their symbiotic zooxanthellae, which require sunlight for the process of photosynthesis, but the physiological relationships of zooxanthellae and their coelenterate hosts are in general less clearly understood in the octocorals than in the madrepores, so the cause of the bathymetricphotic correlation cannot be stated in general terms. Obviously, the vertical distribution of those octocorals that are dependent upon their zooxanthellae for nutrition is governed by the physiological requirements of the algae. In those octocorals that are nutritionally independent of their zooxanthellae (as appears to be generally the case among scleractinian corals) other ecological factors must limit bathymetric distribution. In the West Indies, almost all of the shallow-water octocorals, which represent 38% of the total known fauna, belong to the two families Plexauridae and Gorgoniidae. Very few members of these families extend downward below 25 fathoms, and very few members of the deep-water families venture into water shallower than this. In the East Indies, where a rich tropical alcyonarian fauna exists, 59% of the species taken by the ‘Siboga’-Expedition lived in depths shallower than 50 meters, but this fauna is inordinately rich in groups poorly represented in the West Indies, where 85% of the species are gorgonaceans. In both regions, somewhat more than 40% of the gorgonaceans occur in depths less than 50 meters. The alcyonarians are an important component of the reef community, perhaps more so in the West Indies than elsewhere in the tropics because of the great profusion of a few conspicuous forms in the reef habitat. They provide shelter and sustenance for a wide array of casual associates, epizoa, commensals, and parasites, ranging from other coelenterates to fishes. Moreover, when they die they liberate great quantities of calcareous spicules which are then available for incorporation into the general mass of the reef. The alcyonarian fauna of the warm parts of the western Atlantic shows a high degree of endemism and only indistinct subdivision into smaller faunal regions. It is possible to distinguish a Carolinian fauna occupying the southeastern coast of the United States, with part of its species occurring only along the Atlantic coast and part of them with isolated populations in the northern Gulf of Mexico. At least three species follow the continental coast more or less continuously from the Carolinas to Brazil. This is basically a continental fauna and its species do not range out into the West Indian islands. The fauna of the West Indies is essentially an insular fauna and it suffers depletion wherever it invades continental coasts. The largest number of reef dwelling species seems to occur in the northern islands of the Lesser Antilles, the Greater Antilles, and the Florida Keys. At the present time, more species are known from the last-named locality than from the islands of the Greater Antilles, but it has certainly been more thoroughly explored. Intensive collecting will probably reveal an even larger number of species in the northeastern part of the Antilles. Antillean species extend along both coasts of Florida northward to about the 66°F. minimum surface isotherm, but their number is sharply diminished. A small group of the hardiest species reaches Bermuda, which is the northernmost outpost of the West Indian fauna. Records indicate that the Antillean fauna becomes attenuated also toward the southern islands of the Lesser Antilles, and the Leeward Group along the coast of South America has a fauna comparable in many respects with that of Bermuda. However, the fauna of Bermuda is restricted by the low temperature of the water during midwinter (66°F), a limiting factor that does not exist at the low latitude of the Leeward Islands. The fauna must instead be restricted by other ecological factors, perhaps imposed by the proximity of the continental coast. The alcyonarian fauna of the reefs of Brazil, although composed largely of West Indian genera — Plexaurella, Muriceopsis, Lophogorgia — shares few species, perhaps no more than three or four, with the Antillean region to the north, and is probably the most distinct of the subregions of the western Atlantic. Within the broad limits of the warm western Atlantic fauna 1 region, extending from Bermuda south to Brazil, we can distinguish an insular Antillean fauna centered in the northeastern part of the Antilles; a continental Carolinian fauna along the southeastern Atlantic seabord, some of its species with disjunct populations in the Gulf of Mexico and some following virtually the entire coastline from the Carolinas to Brazil; and a Brazilian fauna extending northward along the South American coast as far as Trinidad. The presence in the West Indies of Alcyonarian genera known also in the tropical Indo-West Pacific can be explained only on the basis of former faunal continuity. The presence of a small amphi-American element clearly points to the existence of a continuous East Pacific-West Atlantic (or trans-American) fauna during the past, and the high level of endemism in the West Indian region suggests a subsequent rapid development of a new fauna from remnants of the old, left behind after closure of the Central American seaways. The distribution of modern alcyonarians corroborates the former existence of a great equatorial sea, the Tethys, that permitted circumtropical distribution of marine animals, which geology tells us existed during much of Earth’s history between the Cambrian and the Tertiary.