Along the coast of St. Martin, St. Eustatius and Saba the rocks above sea-level often show a number of differently coloured zones. This is clearly visible when the coast over a larger distance is formed by one type of rock, as for instance on Saba. In many places a light-coloured belt is seen above the zone with algal growth; still higher the rocks are dark-coloured (Fig. 11, 17 and 22). The uppermost part of the light-coloured zone indicates rather exactly the height of the waves at low tide (splash zone). The darkcoloured zone is wet from finely dispersed water (spray zone). Sometimes differently coloured zones are formed (Fig. 13) which may, however, be correlated with the above mentioned ones on account of their organisms. Below the algal zone on open coasts hardly any other zones can be discerned, since the various communities of plants and animals form here a mosaic-like pattern. On rocky bottoms in the sublittoral region next to the lithophilous algae many corals are present. When the rocks are covered with a thin layer of sand, a rich fauna of Gorgonids may be present; when this layer becomes thicker, the horny corals are replaced by seagrasses and psammophilous algae. From our data the following facts about the rocky coasts, from the supralittoral margin to the uppermost part of the sublittoral region, are obvious. A zonation for molluscs is distinctly visible. Tectarius tuberculatus and Littorina ziczac are found in the spray zone, which is usually recognizable by a dark colour, caused by microscopic bluegreen algae. The rocks in this zone as a rule are almost dry. Chitons are usually present at a somewhat lower level, in places which are constantly washed by the waves; generally they indicate the upper limit of the waves at low tide (e.g. Figs. 17 and 21). They are mostly found in a lightcoloured zone. In places where the rocks are not exposed to the sun, Chitons may occur at much higher levels than is normally the case, when, however, the rocks are scoured by sand they are scarce or absent. Nerita tessellata is common among the Chitons, but may also be found at a lower level among the highest algae. Under overhanging rocks, and also in other places without direct sunlight a characteristic algal vegetation is found, mainly consisting of Bostrychia-species and Polysiphonia howei; the Bostrychias at a somewhat higher level than Polysiphonia. Protected by overhanging rocks and reached by the waves at high tide Bostrychia tenella may form a continuous zone at about 10—20 cm above the water level, with Polysiphonia howei immediately below (Great Bay; Fig. 17). In some cases, however, the Bostrychias may be absent (Little Bay near Kay Bay Hill, and near Fort Amsterdam). Other species with a comparable life form may also be found at a high level in shady places, for instance Lophosiphonia cristata (Little Bay) or Loposiphonia spec. of which very large patches were found on St. Eustatius. Next to these species hardly any algal growth is observed in the littoral region of St. Martin, St. Eustatius and Saba: the high air temperatures and the strong sunshine cause a desiccation of all organisms in the tidal zone. Moreover, because of the oscillations of the mean sea-level during the year, the mortality of the highest algal growth may be considerable. In the zone, which is also washed by the waves at low tide, a moss-like algal vegetation may be found, with many intermingled species, such as: Polysiphonia ferulacea, Lophosiphonia cristata, Jania adhaerens, Cladophoropsis membranacea, Cladophora fuliginosa, Centroceras clavulatum, Valonia ocellata, Dilophus alternans, D. guineensis, Hypnea musciformis, H. spinella, Spyridia aculeata, Laurencia intricata, L. papillosa, L. microcladia, L. gemmifera, Caulerpa racemosa, Dictyosphaeria cavernosa, Dictyota spp., Padina sanctae-crucis, P. gymnospora, P. vickersiae, Grateloupia cuneifolia, G. filicina, Pocockiella variegata, Ulva fasciata, Gelidiella acerosa, Amphiroa fragilissima, Sargassum Stichothamnion antillarum, Pterocladia pinnata, spp., and Wrangelia argus. Combination of species may vary from place to place. On flat rocks, for instance beachrock plates, such species as Polysiphonia ferulacea, Lophosiphonia cristata, Jania adhaerens, Centroceras clavulatum may be predominant. On protruding rocks, as for instance coral limestone, Laurencia papillosa and also Padina sanctae-crucis may be abundantly present. This moss-like algal growth may be found in many places along the coast of the islands, particularly in places with strong wave-action. When the boulders rest in a sandy bottom and much sand is carried by the waves, in several places on the coast of St. Martin a vegetation has developed, consisting of Chondria tenuissima and Digenea simplex, together with a number of species partly mentioned already when describing the moss-like algal vegetation, for instance Padina sanctae-crucis, P. vickersiae, Bryothamnion triquetrum, Laurencia microcladia, L. papillosa, L. poitei, Polysiphonia ferulacea, and Sargassum spp. A clear distinction cannot be made between these two algal vegetations which grow about the same level, as the amount of sand carried by the waves varies from place to place. A number of algal species is very sensitive to the scouring sand: Pocockiella variegata and crustforming Lithothamnia are for instance absent in these places. A number of the algae present under these circumstances has adapted itself to this special habitat, for instance in developing long trichoblasts, which provide extra protection to the growing tips. In a few places a distinct zonation of the brown algae is observed, e.g. in Little Bay off Kay Bay Hill. Here zones are present of respectively Turbinaria turbinata (in the deeper water), Sargassum spec. and Padina sanctae-crucis (in the highest places). These species are found on horizontal or gently sloping places. Other kinds of algae, e.g. Laurencia papillosa and L. microcladia are mostly found on vertical rocks, together with Pocockiella and the crustlike Lithothamnia, which form together a mosaic, nearly completely covering the rock. Apparently the slope of the rock surface determines the establishment of these species. Our observations confirm that Turbinaria and Sargassum prefer exposed places, and Padina less exposed habitats. In many places crustlike Lithothamnia are numerous. On open coasts they may cover almost the entire rock surface below water level. Under overhanging rocks they may be found as high as the Chitons. In exposed places they form rock-covering mosaics together with Pocockiella; in protected places Lithothamnia are predominant; both species are absent in the proximity of corals. A zone of barnacles was observed in a few places. In most cases they are found on the flat upper side of rocks, together with Chitons and the highest algae. Echinometra lucunter is abundant in the zone of the highest algae, but may also occur at a lower level, to a depth of about 50 cm. In deeper water other species of sea urchins are found, e.g. Diadema antillarum, generally in large numbers, both on sandy and on rocky bottoms, and Lytechinus variegatus, mostly solitary in seagrass vegetations. The richest algal vegetation is found on moderately exposed coasts. When the swell is heavy, it tends to impoverish the flora, and the plants become short and compactly built, but even then they may become detached. In quiet water which is insufficiently renewed, the vegetation may be poorly developed too (cf. Guana Bay versus Baie de la Grande Case). In spite of the fact that the significance of the subsoil for marine organisms is fundamentally different from that for land-plant life, the type of rock may still be of influence on the algal vegetation: the possibilities for attachment may be different; niches or small rock pools may offer special habitat conditions. On smooth doleritic rocks in Great Bay near Fort Amsterdam, the algal vegetation for nine-tenths consists of Chaetomorpha media. Normally this species is firmly attached with a branched system of rhizoids, which is much less the case on smooth doleritic boulders where other algae are almost absent. The rocks of the Point Blanche formation also offer special conditions for attachment (Little Bay off Kay Bay Hill). When niches are formed, as for instance in coral limestone or the limestone cliffs of the Low Lands formation, favourable conditions are offered for sciophilous algae, such as Bostrychia and Polysiphonia howei. In tropical countries the difference in water absorption between porous course-crystalline rocks and fine-crystalline rocks is less reflected in the level in which the algal zones of the littoral region are found than in temperate regions (DEN HARTOG, 1959, p. 29). This is due to the high air temperatures, and fluctuations in mean sea-level. In the sublittoral region, different communities of plants and animals have developed in a certain relation to the type of bottom. Where the rocky coast forms a horizontal platform at a depth of a few meters, this may harbour several corals and Gorgonids, and also a number of lithophilous algae such as Amphiroa hancockii, Galaxaura spp., and Halimeda discoidea. In some localities corals, such as Millepora alcicornis, Eusmilia fastigiata, Porites porites, Acropora cervicornis, A. palmata, Agaricia agaricites and Montastrea are found up to rather high levels. Although corals are of general occurrence on St. Martin, St. Eustatius and Saba, the beaches of coral debris, well-known on Bonaire and Curaçao, are not present. This possibly may be explained by the fact that the sea around St. Martin is rather shallow, and the bottom over large areas consists of sand. When the seabottom consists of sand, the littoral region and the uppermost part of the sublittoral region may be devoid of algae. At a greater depth where the sandy bottom is more stable, vegetations of Thalassia testudinum and Syringodium filiforme and also of algae may develop. Especially Thalassia is important for accumulating sand: the Thalassia fields gradually rise and finally may protrude 30—50 cm or more above the surrounding seabottom (cf. Simson Bay and Mouth Piece Bay). In seagrass vegetations many species of algae are found; particularly representatives of the order Siphonales may be fairly abundant, also in bare sandy bottoms which are somewhat settled. A seagrass vegetation with algae is not only found on open sandy coasts but also in the lagoons, especially at their shallow entrances, where the water is more quiet than in open bays. The presence of numerous spherical-branched Lithothamnia among the Thalassia vegetation, at a depht of about 40 cm, near the entrance of Oyster Pond, is remarkable. The algal vegetations on the roots of Rhizophora in Oyster Pond and in the entrance of Fish Pond are exactly as described in the literature. In the littoral region Bostrychia binderi and Polysiphonia howei are common; in the uppermost part of the sublittoral region many other species are present, especially Acanthophora spicifera, together with Caloglossa leprieurii, Murrayella periclados and Ulva lactuca. In the rear part of the lagoons, and along the banks of lagoons completely shut off from the sea, the bottom is muddy and the salt content shows remarkable fluctuations. Only a few euryhalinic algal species are found in these shallow pools; especially Batophora oerstedi and Enteromorpha spp., in most cases attached to coral fragments, pieces of wood or shells.