o70 Bulletin American Museum of Natural History [Vol. LIII 
moreover, we have not the necessary material. Since we have discussed 
at some length the general uniformity of the mollusks of Ethiopian flow- 
ing waters and its possible causes, we may as well point out here that the 
fauna of the Congo basin exhibits somewhat more individuality than is 
usual on the continent. This is mainly due to the presence of a fair 
number of elegant Melaniide belonging to the genera Potadoma, Mela- 
noides, and Cleopatra, as well as of Mutelide, especially the curious 
subgenus Chelidonopsis. Potadoma is restricted to J. P. Chapin’s 
Guinean Forest Province of his West African Subregion (see p. 486), 
while Chelidonopsis is not known outside the basin of the Congo.! 
Nile River System? 
The Nile is the longest river of Africa and the second longest of the 
earth, coming close behind the Mississippi-Missouri. Taking the Kagera 
as its headwaters, its length from source to sea reaches about 6,500 kilo- 
meters. Its drainage system is, however, much smaller than that of the 
Congo, covering approximately 2,900,000 square kilometers. The volume 
of water it carries is still further reduced, because of its crossing some 
3,200 kilometers of desert country, where it receives no additions, but 
instead loses much through evaporation. Its upper reach, above Khar- 
toum, is known as the Bahr-el-Abiad or White Nile and south of 10° N. 
as the Bahr-el-Jebel or Albert Nile, which drains the Lake Plateau basin 
or the waters of Lakes Victoria (by way of the Victoria Nile), Edward (by 
way of the Semliki), and Albert. Its chief tributaries are the Sobat, the 
Bahr-el-Asrek or Blue Nile, and the Atbara on the right, draining western 
Abyssinia; and the various branches of the Bahr-el-Ghazal system on the 
left. The divide between the headwaters of the Bahr-el-Ghazal and 
those of the Uele-Bomu-Ubangi is generally low, forming a fairly level 
peneplain 800 m. to 900 m. above sea level, with a series of granitic 
residual hills. In this region many of the swamps where the streams 
take their source drain both to the Congo and to the Nile, at least 
1One species of Potadoma, P. liricincta (EK. A . Smith), was originally described from ‘‘ Lake Albert,”’ 
supposedly collected there by Emin Pasha; but it has not been found again in that lake, so that its 
occurrence there is extremely problematical. From the introductory remarks of the paper in which 
it was described, it is quite evident that the specimens were not labeled as to localities. It is more likely 
that the types of this melanian came from one of the forest streams west of Lake Albert, a region actively 
explored by Emin Pasha. 
Potadoma liricincta major (J. Thiele) was later described from the region between Beni and Boga. 
Schubotz, who collected the specimens, travelled to the west of the Semliki and it is possible that he ob- 
tained the snail from one of the forest brooks flowing to the Semliki. It may, however, have come equally 
well from the headwaters of one of the tributaries of the Ituri. An examination of the map of this region 
published in the Report of the Land Mollusks (1919, Bull. American Mus. Nat. Hist., XL, p. 17) will 
show how close the Congo-Nile divide here runs to the valley of the Semliki. 
_ We conclude that on the whole the evidence that the genus Potadoma occurs anywhere within the 
basin of the Nile is not yet convincing. 
peee Laon) H.G. 1906. ‘The physiography of the River Nile and its basin.’ (Cairo), viii +411 
Pp., s. 
