2 LIEUT.-COL. GODWIN-AXJSTEN ON SOCOTRAN SHELLS. [Jan. 16, 



1. On the Freshwater Shells of the Island of Socotra collected 

 by Professor I. Bayley Balfour. By Lieut. -Colonel 

 H. H. Godwin- A.USTEN, F.R.S., F.Z.S. &c.--Part III. 



[Eeceired January 16, 1883.] 

 (Plates I. & II.) 



The shells now treated of belong to the genera Planorbis, Hy- 

 drobia, and Melania. The first is the only representative of the 

 LimnceidcB in Socotra, for the very generally distributed genus Lim- 

 ncea 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 Mdania are well known shells 

 with an extended range, and have been often fig\ired in various works, 

 yet they vary much in form, coloration, and sculpture with change 

 of conditions. I have therefore given figures of their Socotran repre- 

 sentatives. I think, when we are trj'ing 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 the 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 of such pieces of water 

 or marshy ground — the one conducted into new cliannels, 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 de- 

 ductions made now on the distribution of species as connected with 

 the former outlines of land and sea. If mere lists of a fauna, with 

 perhaps meagre deescriptions onl}-, be drawn up, and a species 

 become extinct and the original collection destroyed, how easy is it 

 to throw doubt on the authenticity and correctness of the record, or 

 the identification of the particular species. When 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 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 cocJcburni may be also African ; but it is a 

 form of a group of that genus which has a greatly extended range 



