XXXU BOTANY OF SOCOTRA. 



Great interest always attaches to the land and fresh-water mollusca of a 

 large and ancient island, and in this feature Socotra is not disappointing. 

 Lieut. -Col. Godwin-Austen records forty-eight species in our gathering from 

 Socotra, and of these a large portion are endemic. Amongst the land-shells 

 some of the genera have very instructive distribution. Thus Otopoma is 

 restricted to the east African islands and Arabia ; Lithidion has the same area, 

 but extends to India; Cydotopsis is represented outside Socotra only in India 

 and the Seychelles ; whilst Tropidophora is known from Madagascar alone. 

 Writing of the land-shells, Lieut.-Col. Godwin- Austen says:" — "Judging from 

 the land-molluscan fauna of Socotra, there is strong evidence that the island was 

 once directly connected with Madagascar to the south. We know the great 

 antiquity of that island; and it is not unreasonable to suppose that in Socotra, 

 the Seychelles, Madagascar, and Rodriguez, we have the remnants of a very 

 ancient more advanced coast line on this western side of the Indian Ocean, 

 which line of elevation was probably continuous through Arabia towards the 

 north. With an equally advanced coast on the Indian side, the Arabian Sea 

 would, under these conditions, have formed either a great delta, or narrow arm 

 of the sea, into which the line of the Indus and Euphrates drained. Such 

 conditions would have admitted of the extension of species from one side to 

 the other, which the later and more extensive depression of the area, as shown 

 in Scinde, afterwards more completely shut off." 



And again he says,t when dealing with the fresh-water mollusca, — " The 

 fresh-water 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 the Mascarene Islands to the 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 

 in time and area. It seems remarkable that four fresh- water shells of common 

 and abundant Indian species, only one hitherto known from Africa should be 

 found isolated in Socotra ; and this, I think, is another point in evidence of 

 the area of the Arabian Sea as far south as a line joining Madagascar and 

 Ceylon having been once, to a great extent, dry land, receiving the drainage of 

 the surrounding mountain-ranges, of which Socotra formed a portion of the 

 western watershed and the limit of its fresh-water fauna, this watershed being 

 then continuous with the Jebel Yafai and the highlands of Arabia." 



* H. H. GodwinAusten : On the Land-Shells of the Island of Socotra, collected by Prof. Bayley 

 Balfour, — Part I. Cyclostomacese, in Proc. Zool. Soc, Feb. 1, 1881, pp. 351-258, with plates xxvii. 

 and xxviii. ; and Part II. Helicacea, in Proc. Zool. Soc, June 21, 1881, pp. 802-812, with plates 

 lxviii. and lxix. 



t H. H. GodwinAusten : On the Fresh-water Shells of the Island of Socotra, collected by Prof. 

 Bayley Balfour, in Proc. Zool. Soc, January 16, 1883, pp. 2-8, with plates i. and ii. 



