1902.] AND ANCIENT GEOGRAPHY. 301 



farther in Algiers as far as Oran. 1 It is a remarkable fact that this 

 subgenus is entirely absent from Africa proper, i.e., the part of it 

 that lies to the south of the Sahara Desert. 



The subgenus Potamonautes, on the contrary, has its chief centre 

 of distribution in tropical Africa. It has been found, beginning at 

 Liberia, all along the western coast as far as Mossamedes. It is 

 found in the interior, in the region of the upper Zambesi (Kazun- 

 gula), extends over Transvaal to the Cape Colony, and northward 

 all along the eastern coast (Natal, Mozambique) to German East 

 Africa. Also in the eastern part of the interior it is represented, 

 for instance, in the headwaters of the Nile (Victoria Nyanza) and in 

 the Somali country. From the upper Nile it extends down the 

 Nile valley as far as Bahr-el-Gebel in the Egyptian Soudan. It is 

 also found on the Island of Socotra and in Madagascar, although 

 the species of the latter island do not seem to belong to the typical 

 form of this subgenus. 2 



1 A. Milne-Edwards reports a species that is identical with an Indian (P. le- 

 schenaudi (M.-E.)) from Mauritius : this locality, however, lacks confirmation. 

 As regards the Madagassian species of Potamon, their systematic position is 

 doubtful, and they possibly do not belong to this subgenus. Compare next note. 



2 Three species of Potamon are known from Madagascar. P. goudoti (M.-E.) 

 (see A. Milne-Edwards, 1869, P- I 7 2 > PI- 8» Fig. 4) is a peculiar form, but its 

 postfrontal crest distinctly points to Potamonautes. A. Milne-Edwards compares 

 it with P. obesum A. M.-E. from Zanzibar, and indeed it seems to be closely 

 related to it. The latter species is also an abnormal type of Potamonautes, and 

 forms with several others a group that is peculiar to East Africa ; but there is 

 no reason to separate this group from Potamonautes, and thus we may safely 

 regard P. goudoti as a Potamonautes. The second species is P. madagas- 

 cariense (A. M. E.) {Ann. Set. Nat. Zoo/., Ser. 5, Vol. 15, 1872). As to this 

 form, the diagnosis of which is very brief, and which has not been figured, its 

 author says that it is a true 7 ' helphusa (i.e., subgenus Potamon"), but this seems 

 hardly correct according to the description of the postfrontal crest, which is said 

 to be simply interrupted in the middle, while the median parts of it are not 

 advanced beyond the rest. This would better agree with Potamonautes. The 

 third species is regarded by A. Milne-Edwards {Ibid., 1872) as the type of a 

 separate genus, Hydrothelphusa (H. agilis A. M.-E.). This genus is said to be 

 chara:terized by the flat carapace, which is scarcely dilated and almost quad- 

 rangular, and by the horizontal front. The postfrontal crest is distinct and 

 interrupted. Since no figure is given, it is hard to form an opinion as to the 

 relation of this form to others, but it seems to be very peculiar. 



Thus it seems that the Madagassian species of Potamon show, in som e 

 respects, a distinct relation to East Africa and the subgenus Potamonautes, 

 while in others they appear quite peculiar. (This is opposed to the opinion 

 expressed by myself in 1901, p. 1290, footnote.) 



