1883.] LIEUT.-COL. GODWIN-ATJSTEN ON SOCOTRAN SHELLS. 3 



in time and area. It seems remarkable that four freshwater 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 laud receiving the drainage of the surrounding mountain- 

 ranges, of which Socotra formed a portion of the western watershed 

 and the limit of its freshwater fauna, this watershed being then 

 continuous with the Jebel Yafai and the highlands of Arabia. 



Herr von Martens, the recorder of the Molluscan portion of the 

 'Zoological Record' for 1881, does not quite agree with me in connect- 

 ing Socotra with Madagascar, considering the species I placed in Tro- 

 pidophora to belong rather to Lithidion. On looking again at these 

 shells, the form of the operculum of T. socotrana is certainly similar 

 to that of Otopoma ; but that of Lithidion is nearer to Cyclotopsis and 

 Tropidophora (Arabia and Socotra). 



1 hope soon to be able to examine the animals of these shells, as 

 Professor Balfour placed some of those collected in spirits, and others 

 were alive when the}'^ reached England. 



Fam. LiMN^iD^. 

 Planorbis exustus, var. maculatus. (Plate I. figs. 1, la, 16.) 



Shell, umbilical region slightly concave, the apical very slightly 

 depressed ; sculpture, very fine regular transverse ribbing ; colour 

 umber-brown, passing into white, with two or three broadish trans- 

 verse bands of darker brown, more apparent in bleached specimens ; 

 spire very slightly depressed ; whorls 4, the last increasing rapidly ; 

 aperture widely ovate, descending below and slightly rising above 

 the level of the body-whorl ; peristome thin, margins united by a thin 

 deposit on the body-whorl. 



Size : — 



Major diam. 12'5, minor diam. 9'0, alt. axis 4*5 millim. 

 „ 0-49, „ „ 0-35, „ 0-18 inch. 



This species is about the size and has somewhat the form of P. 

 madagascariensis, Ed. Smith, figured and described in the P. Z. S. 

 1882, from Lake Itasy, 



Compared with a large series of P. exustus from different parts of 

 India, it is rounder on the periphery and smoother than the majority ; 

 and none of the Indian examples exhibit the distinct bands of darker 

 colour on the last whorl. 



This shell and others of the genus are no doubt really sinistral ; 

 but I have figured it as dextral, this being more convenient for com- 

 parison with almost all figures hitherto given of this genus. 



Planorbis socotrensis, n. sp. (Plate I. figs. 3, 3a, 3S, 3c.) 



Shell minute, discoid, apical and basal sides equally concave; 



sculpture, obliquely striate with lines of growth, otherwise smooth 



with very minute pitting or malleation ; colour pale ochraceous ; 



suture well impressed ; whorls 3, flat on the periphery, angular 



1* 



