554 



DR. W. A. CUNNIISTGTON ON THE 



that fewer species occur in the smaller than in the lai-ger lakes. 

 With tiie exception of Tanganyika, the lakes are arranged in 

 order of size, and the figures for the species are as follows : — 

 Tanganyika 17, Victoria 18, Nyasa 13, Albert 8, and Edward 5. 

 Regarding Tanganyika as a lake apart, the remaining figures fit 

 in satisfactorily with this suggestion. 



The features which are characteristic of the difterent lakes may 

 be summed up in a few words. Tanganyika alone contains 

 endemic Lamellibranch genera. SphceriLim, Eiqyera, and Spatha 

 are unrepresented, but a number of endemic species of Unio are 

 known. — Victoria Nyanza, with the largest total of species, 

 displays, notwithstanding, little of interest. Most of the genera 

 are represented, except those peculiar to Tanganyika, and a series 

 of forms belonging to I'iijhcerium and Unio are to be observed. — 

 Of Nyasa there is likewise little to record. The lake only 

 contains species from the four well-known genera Corhicula^ 

 Unio, Mutela, and Spatha, 3 endemic types of the last-mentioned 

 being an outstanding fea.ture. — Both Albert and Edward Nyanza 

 are similar in type to Nyasa, but contain a representative of 

 Sphterium and not SpatJia. 



From this survey of the Lamellibranchs, the following points 

 emerge : — No thalassoid types occur in Tanganyika, but that lake 

 contains a number of endemic species, as well as three endemic 

 genera. All the lakes show a high percentage of endemic forms, 

 but in most cases these are merely species belonging to widely 

 distributed genera. While Tanganyika does not exhibit in this 

 instance so remarkable a, series of unique genera and species as 

 in the case of the Gasteropods and other groups, it retains never- 

 theless a distinctness from the remaining lakes quite in keeping 

 with its general character. 



Macrura. 



The only Macrurous Crustacea v/hich appear to occur in the 

 big African lakes are the jDravvns, these being quite common 

 types in the fresh-v/aters of the tropics. An examination of the 

 forms now known to exist in the lakes of Africa reveals, how- 

 ever, many points of intei'est. In the first place, no fewer than 

 twelve species have been discovered in Tanganyika, of which ten 

 were obtained for the first time by the Third Tanganyika 

 Expedition. This is in itself a large number of different species 

 to be found within the limits of a single lake, but the interest is 

 greatly increased when it is realised that all these forms occur 

 in Tanganyika alone. This is the only well represented group in 

 which such is the case. Again, with the exception of a single 

 species of the well-known fresh-water genus Fcdcemon^ all the 

 genera are equally to be regarded as endemic. 



