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Show 2 LIEUT.-COL. GODWIN-AUSTEN ON SOCOTRAN SHELLS. [Jan. lb» 1. On the Freshwater Shells of the Island of Socotra by Professor I. Bayley Balfour. By Lieut.-Colonel H. H. GODWIN-AUSTEN, F.R.S., F.Z.S. &c-Part III. [Received January 16, 1883.] (Plates I. & II.) The shells now treated of belong to the genera Planorbis, Hydrobia, and Melania. The first is the only representative of the Limnceidov in Socotra, for the very generally distributed genus Lim-neea appears certainly absent. The freshwater shells brought home by Professor Balfour are well represented numerically ; and a great number of young specimens occurred caught up in the water-plants that were collected ; but all are referable to the above genera, and not a single bivalve of any kind was detected among them. Although three of the species of Melania are well known shells with an extended range, and have been often figured in various works, yet they vary much in form, coloration, and sculpture with change of conditions. I have therefore given figures of their Socotran representatives. I think, when we are trying to slowly work out the causes of the distribution of species over certain areas, we cannot be too particular, too minute, and too exact with the species that we are now collecting, and more particularly with island forms. I do not imagine that in Socotra tbe more or less stagnant pools are large or numerous, or the streams of great extent; and this must be the case with many small islands. The formation of a Military or a Coaling-station is very apt to lead to the destruction ot such pieces of water or marshy ground-the one conducted into new channels, the other drained for sanitary purposes, destroying the original molluscan inhabitants together with many of the plants, insects, and other forms of life. Again, the introduction of plants with the occupation of islands by new races, leads to the transport of species from other countries ; and as time goes on the history of such aided emigration is lost, and there will be a tendency to weaken original deductions made now on tbe distribution of species as connected with the former outlines of land and sea. If mere lists of a fauna, with perhaps meagre deescriptions only, be drawn up, and a species become extinct and the original collection destroyed, how easy is it to throw doubt on tbe authenticity and correctness of the record, or the identification of tbe particular species. W h e n drawings are added there is less possibility of such doubts arising. The freshwater shells we have before us have certainly more of an Indian character than an African one; and, again, as I pointed out in a previous paper, they extend to Madagascar and tbe Mascarene Islands to tbe south. In fact the only species in the present series that has an African habitat is the extremely wide-spread Melania tuberculata. Planorbis cockburni may be also African • but it is a form of a group of that genus which has a greatly extended range |