FAUNA OF THE AFRICAN LAKES. 



553 



Tanganyika with 14 endemics out of 17 leads the way as far 

 as endemic Lamellibranchs are concerned (82 per cent.), Lake 

 Victoria coming near with 12 out of 18 (66 per cent.), while 

 Nyasa has 7 out of 13 (54 per cent.). Even the smaller lakes 

 have each two endemic species. It will be observed that the 

 proportion of peculiar forms is very high in this group also ; 

 indeed, in some instances it is higher than in the case of the 

 Gasteropods. Endemic genera are known only from Tanganyika, 

 three being retained in the list out of a larger number distin- 

 guished by Bourguignat. The genus Moncetia of Bourguignat, 

 inserted in the table as endemic, though not definitely rejected, 

 is nevertheless considered by Smith as only doubtfully separable 

 from Sjxitha (170, p. 101). 



A brief survey of the list of species will sulfice. Corhicula 

 radiata, a representative of the Cyrenid^ie, is the only form which 

 is on record from all the lakes concerned. It is a widely distri- 

 buted African type, being known from other parts of East Africa 

 and from the valley of the Nile. — The family Sph?eriidie is 

 represented by the genera Sphcerium and Eupera. It is some- 

 what strange that the four species enumerated are all found in 

 Lake Victoria, but not in Tanganyika or Nyasa. Sphmrimii 

 nyanzce is knou n from Victoria, Albei-t and Edward Nyanzas, as 

 well as from other parts of the continent, but it is associated in 

 Victoria with two additional species which are peculiar to that 

 lake. Eupera parasitica^ which is a Nilotic and N. African form, 

 is at present only recorded from Victoria Nyanza. — In the family 

 Unionidfe there are associated with the extensive genus TJiiio 

 itself only the two closely related genera Burtonia and Bra?^zcm, 

 which are confined to Tanganyika. No fewer than 29 different 

 species of Unio are enumerated, not one of which is on record 

 from more than a single lake. Tanganyika exhibits 8 and Lake 

 Victoria 7 endemic species. Of 7 types in Nyasa 3 are endemic ; 

 of 5 in Lake Albert 2 are endemic. Lake Edward, lastly, 

 contains 2 forms, both of which are endemic. — In addition to 

 three genera of wider distribution, the Mutelid?e contains the 

 doubtful genus Moiicetia, to which reference has already been 

 made — a genus described as peculiar to Tanganyika. Mutela 

 nilotica is the only species in the family recorded from more than 

 one lake, it being found in both Albei't and Edward Nyanza as 

 well as in Egypt. Each of the bigger lakes exhibits a single 

 endemic form of Mutela. The genus Pleiodon is only represented 

 by P. spekei, conlined to Tanganyika — it is one of the species 

 named by Woodward from Speke's original collection. Spatha is 

 unrepresented in Tanganyika and the smaller lakes, but a series 

 of three endemic forms is known in Nyasa. — Lastly, ^Etheria 

 eUlptica, sole representative of the ^theriida?, occurs in Tangan- 

 yika and Victoria Nyanza., but has not been recorded from the 

 other lakes, although it is widely distributed in Tropical Africa. 



While a considerably smaller number of forms is involved here 

 than was the case with the Gasteropods, there is yet an indication 



Piioc. ZooL. Soc— 1920, No. XXXVII. 37 



