184 MR. EDGAR A. SMITH ON THE MOLLUSCA [Mar. 6, 



features which are only developed in adult shells. Having broken 

 up one specimen, I failed to find an operculum. 



h. Non-Thalassoid Species. 



1. Limn^a natalexsis Krauss. 



Hah. Swamp at Mbete, south end of lake. 



2. Ancylus tanganyicensis, sp. n. (Plate X. figs. 17, 18.) 



Shell very small, roundly ovate, moderately elevated, thin, 

 brown, finely radiately striated across the concentric lines of 

 growth ; apex obtuse, boss-like, circvimscribed, radiately striated, 

 subcentral or a tidfle towards the light ; interior glossy, exhibiting 

 through the semitransparency of the shell the external sculpture. 

 Length 2| millim., diam. 2, height 1. 



Hah. On a stone dredged in a few fathoms in Niamkolo Harbour, 

 south end of lake. 



This is the first and only species of Anclyus known from the 

 lake. The genus, however, occurs both in the north and south of 

 the African continent, but with the exception of A. stiMmanni 

 Mrti'tens, from the Victoria Nyanza, no species have been recorded 

 from the great lakes. Being so small it is possible they may have 

 been overlooked. 



Prof. Gwatkin, who has very kindly examined the i-adula, 

 observes: — "As I expected, it belongs to the A. parallel as type, 

 which I have from S. Africa, Australia, and North and South 

 America. To it belong my ' Gundlachias ' from Tasmania and 

 New Zealand." 



3. Plaxorbis sudanicus Martens. 



JIab. Swamp at Mbete, south end of lake. 



4. Neothauma tanganyicense Smith. 



Hab. Kituta, Kala, Moliro, Sumbu, Kalambo. 



Keeled and non-carinate specimens were found together at the 

 south end of the lake by Dr. Ounnington, so that Mr. Moore's 

 idea of the local distiibution of this species appears to be, in a 

 measure, incorrect. 



5. Ampullar: A ovata Olivier. 

 Hab. South end of lake. 



6. Unio burtoni Woodward. 



Hab. South end of lake, and Kala at south end of east coast. 



7. BuRTONiA tanganyicensis (Smith). 



Hab. Kombe, east coast below the middle, and south end of the 

 lake. 



8. Pleiodon spekei Woodward. 

 Hab. Sumbua, east coast. 



