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Show 248 MR. E. A.SMITH ON THE [Apr. I, washed ashore on the windward or south side of the island. These specimens will be enumerated in an Appendix, as they cannot be regarded as belonging to the St. Helena fauna. In nearly every instance in which it has been possible to associate them with known species, they prove to be South-African forms, thus clearly showing that they have been drifted northwards from the Cape by the prevailing south-east trade-winds and oceanic currents. Capt. Turton observes in his notes that some of them were alive when taken, and this was generally the case when the " Sea-horn " was only recently washed up, or was secured from a boat. Notwithstanding this fact, it is remarkable that scarcely any (exclusively) South-African species appear to survive and become established at St. Helena; indeed, Gadinia costata is the only species in this collection, not found on " Sea-horn," the distribution of which has hitherto been restricted to South Africa. A few species such as Triton o/earium, Triforis perversa, Cingulina circinata, Saxicava arctica, Mytilus edulis, M. magellanicus, Arca domingensis, Pinna pernvla, and perhaps one or two others, are found at both localities, but they mostly have a wide distribution. As it is seen that many species are drifted from the Cape to St. Helena, the question arises whether some of those dredged by Capt. Turton, or found by him and others upon the shore, may not have become detached from the floating seaweed. In one or two cases it is pretty certain that this has occurred, as specimens of Mytilus magellanicus and M. edulis (?) were obtained alive attached to floating weed and also dead upon the shore. Two dead specimens of Patella compressa, a well-known Cape species, were also collected on the shore, there being every probability of their having been carried there attached to seaweed. The molluscan fauna of St. Helena appears most to resemble that of the West Indies ; for, of the known species 1 in this collection, just fifty per cent, are common to the two localities. About five-and-twenty species, or thirty per cent., are identical with Mediterranean forms, and about half a dozen occur at all three localities. About thirteen species are also met with on the West- African Coast, between the Gulf of Guinea and Morocco. What proportion of species are common to St. Helena and the west coast of Africa, south of Guinea, it is difficult to ascertain at present, as comparatively little is known of the Mollusca of that part of the coast. However, in Dunker's list of shells from Lower Guinea, eight species are quoted which are common to St. Helena. The similarity between the fauna of St. Helena aud that of the West Indies is undoubtedly, in a great measure, due to oceanic currents. According to various maps an important current flows from near the centre of the South Atlantic past Ascension Island along the north coast of South America to the West Indies, a return current passing in an easterly or south-easterly direction towards the Gulf of Guinea. These and the great Gulf-stream in all probability have 1 Pelagic forms are not included. |
