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118 H. PETER HALLBERG AND REINHOLD IV ARSSONFig. · 3. Shell-bearing sandy shorein a sheltered, east-facing cove,Trosso Island, Tanum, N. Bohuslan.Near the shore scattered Elymusarenarius and M inuartiapeploides; between decumbent Juniperusshrubs Tortulo-Phleetumarenarii; close to the typicallysculptured granite hills Helictotrichetumpratensis; on rocks inforeground Sedo-Tortelletum. Moderategrazing influence, as indicatedby these communities. Thesite is on the border of spontaneouspinewood towards the deciduouswood region. Aug. 25, 1964. PhotoH. P. Hallberg.However, the small remains of Quercus_:_Fraxinus­Ulmus-Oorylus forest are, in many instances,dependent on shell deposits.Especially the archipelago of northern Bohuslanhas a widely distributed and variable vegetation onshell deposits, with a fairly rich composition byspecies. This may, no doubt, be assumed to be dueto circumstances relating to the history of itsdistribution, e.g. the proximity to the abundantlime vegetation on the Oslo Cambro-Silurian (cf.STERNER 1945), and, further, to the existence ofextensive sand deposits containing shells, and tothe dry climate of the archipelago (see above). Insouthern and middle Bohuslan, sand containingshells is rare in the coastal area.The shell banks have almost all been exploitedfor their content, and many have been entirelyobliterated. Others have been dug out right downto the clay layer, or to the subsoil water-level. Theshell banks are generally small and isolated inpockets in the ground surface. Their vegetation isto a great extent heterogeneous. They often formrefuges for rare plants (S. NILSSON 1940, 1945).ToRTULO-PHLEETUM.-In the archipelago, thecommunity 1,'ortulo-Phleetum arenarii (BRAUN­BLANQUET & DE LEEUW 1936, p. 365), which be-longs to the calciphytic vegetation, inhabits thesand of a zone above Elymo-Ammophiletum (seeabove), being generally barely discernible and of afragmentary composition. The many whorls ofleaves on the oversanded stems of Taraxacumspecies (Erythrosperma) indicate that the substratumis mobile. Tortulo-Phleetum in Bohuslan,when typically developed, is inhabited e.g. byAnthyllis vulneraria ssp. Linnaei, Galium verum v.litorale, Phleum arenarium, . Sedum acre, Tortularuraliformis, and the rare fungus Tulostoma brumale(cf. FRISEND.AHL 1926, p. 127). The associationcomprises several subassociations related to theFestuca ovina - Galium verum v. litorale associationand to Helictotrichetum pratensis (see below). Avariant rich in species on humous shell deposits is,as a rule, dominated by Ditrichum flexicaule, (cf. 0.ANDERSSON & W ALDHEIM 1946, p. 105).SEDO-TORTELLETUM.-A Sedo-Tortelletum (cf.ALBERTSON 1946 a, p. 45, TUXEN 1951, p. 166)develops on shallow shell deposits in crevices and onflat rocks. Some of the species belonging to thiscommunity are Sedum album, Ditrichum flexicaule,Tortella tortuosa, and more rarely Oladonia symphycarpiaand Toninia coeruleonigricans. Communities,with a similar composition, are found near the shoreActa Phytogeogt·. Suec. 50