33 ° 
THE TANGANYIKA TROT LEM. 
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 , Chytra 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 Stroinbus , 
Xenophora ( Onustus), Natica and Cerithium. The Gymno- 
leamatous Polyzoan Arachnoidia 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 Ire 
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 Ire 
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. 
