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Show 1 9 0 5 . ] CAPE VERDE MARINE FAUNA. 1 7 9 these spaces are the habitat of a rich fauna; they are often practically filled with Lamellibranchs and free-living Gastropoda, while Polychseta, Sipunculids, small Crustacea (especially Amphi-poda), Nemertines, and even Centipedes can be washed out in great abundance. Boring Lamellibranchs (Lithophagus sp. ?) are common, sometimes astonishingly so, but Polyclueta and Sponges of this habit are far rarer than in the pure Alga or Coral of the shore-pools and bottom below tide-level. Further north still, where the surf breaks strongly, the surface of the incrustation becomes more or less bare of the mossy weeds and more or less foliaceous, and on breaking into its smooth portions the proportion of Vermelus-tv\)QS is found to have greatly decreased. Further on are inaccessible rocks, covered with the light brown branched nullipore described above. In some localities, e. g. the promontories just south of St. Vincent Harbour, an Eupsammid coral forms the lower part of the band of incrustation. This is always in a friable condition, and large pieces can be detached by the bare hand. The incrustation is soft but tenacious, so that a crowbar must be driven in several times before a piece can be detached. Indeed, I found the best way of breaking it from the rocks to be by hammering in the blade of a spade. I saw no evidence of pieces being broken away by the waves, so that the causes of the limitation of these growths to masses rarely so much as one foot thick are not evident. Owing to the limitation of the zone to so narrow a band of these steep shores, continued growth would result in the formation of an unsupported shelf, which would at once be broken away by the sea. But I believe the action of boring-organisms to be more important. Physical conditions doubtless are the prime factors in determining the balance of life, and here they seem to have given the predominance to the agents of destruction, while the cold currents from the north account for the subordinate position of the Corals. A. great amount of rock-formation is going on at depths of from 5 to 20 fathoms. Where-ever I have dredged in from 5 to 10 fathoms (St. Vincent, Porto Praya, and Bonavista, text-figs. 26, 25, and 24), nodules of nullipore Lithothamnion * are strewn abundantly over the bottom. From 10 to 20 fathoms two more delicate kinds occur, one being soft and foliaceous, the other consisting of thin and brittle branches. The fate of these I do not know, but of the nodules the great majority are rendered rotten by Sponges and boring Polychfeta, finally breaking down to a grey mud. Among and below these Algfe there is a coarse sand formed almost entirely of a large foraminiferan. This covers practically the whole floor of St. Vincent Harbour between the 5 and 20 fathom lines, but finer sand and mud are plentiful in Porto Praya, and sand of volcanic origin in Bonavista. In St. Vincent the resulting mud from the destruction of the organic rocks appears to be carried to * Herdman's ‘ Pearl-Fishing Report ' contains an excellent illustration of these But in Ceylon the agents o f destruction seem to be different. |