33o THE TANGANYIKA PROBLEM. 



marine in character, and the invertebrates belonging to 

 which are peculiar to Lake Tanganyika ; the ganoids, 

 Polypterus and Protopterus, which I have included in the 

 halolimnic series, are old and primitive forms. Among 

 the Cichlidae which are peculiar to the lake there are, 

 as Mr. Boulenger has shown, forms which appear to be 

 among the most primitive living representatives of the 

 group. The crabs characteristic of Tanganyika were 

 found by Mr. Cunnington to be among the most primitive 

 of the thelfusoid series.* The prawns peculiar to the lake 

 have, as Mr. Caiman remarked, nothing specially primitive 

 about them. But prawns belong to an ancient palaeonto- 

 logical group. The long list of gastropods belonging to 

 the halolimnic group exhibit a diversity of structure which 

 is truly surprising. In the case of Nassopsis, CJiytra and 

 Limnotrochus, we have new and really archaic forms, pos- 

 sessing characters which indicate that they are directly 

 related to the di-branchiate forms. The rest of these 

 molluscs are almost equally peculiar, and seem to be little 

 specialized members of the great marine groups of the 

 present day, which are typified by such genera as Strombus, 

 Xenophora (Onustus), Natica and Cerithium. The Gymno- 

 leamatous Polyzotm Aracknoidia is only known from Tan- 

 ganyika, it belongs to a group, of which only one or two 

 species are known to inhabit fresh or brackish water, while 

 at the same time it is structurally quite different from any 

 of these few which have elsewhere succeeded in main- 

 taining themselves in lakes and streams. 



* With respect to the large Tanganyika crab, Platythelfusa arma/a, it should be 

 pointed out that Milne-Edwards when he described it (see Chapter XII.), remarked that 

 but for the fact that its young went through no metamorphosis, the species would be 

 more readily regarded as a member of several marine groups, and it is obviously open 

 to question whether the fact of its young being without metamorphosis can legitimately 

 be regarded as a distinctive feature. 



