Introduction.—A large collection of non-cichlid fish from Lake Tanganyika, containing 2475 specimens and 57 species, made by the late Dr. Cuthbert Christy, has now been identified. The collection is described in this paper, which forms part of a series attempting to put on record all data relating to the fish of the lakes of Africa. The present work has been undertaken as a preliminary to further ecological studies in the field.

History of Investigations.—Since its discovery in 1858, by Burton and Speke, Lake Tanganyika, having so peculiar a fauna, has been visited by many collectors, who have brought back specimens, including fish, till at the present time there are 76 species of non-cichlids known from the lake.

Physiography.—The drainage-system of Lake Tanganyika is small in proportion to the size of the lake, the main affluent being the Ruzizi River from Lake Kivu, while the only outlet is the Lukuga River, which flows westwards into the Lualaba River, a tributary of the Congo. Little is known about the physical and chemical conditions of the water, except that the depth is very great and that the salinity is rather high, with a preponderance of magnesium salts.

The Fish-fauna and, its Distribution.—A table shows all the species of nonaichlid fish known from Lake Tanganyika, together with their distribution in other parts of Africa. The proportion of endemic forms, 42 out of 76 (55 per cent.) is very high, and most of these belong to groups that inhabit open and well-oxygenated waters and which are not able to penetrate through small streams and swamps to other drainage-systems.

The distribution of the remaining 34 species in Lake Tanganyika is analysed, and the number of these found in each of the various drainage-systems is determined. There are twenty in the Congo system, ten of which are also in the Zambezi system, four others in the Zambezi or East African Rivers, two in Lake Kivu, possibly two in Lake Rukwa, eight in Lake Victoria, and seven in the Nile.

History of the Lake and its Drainage-system.—Günther's and Moore's hypothesis of a marine origin of Lake Tanganyika has become untenable as a result of recent evidence. It is now certain that the lake came into existence in the centre of the continental mass as a result of the great rifting movements, which start in early Tertiary times. The subsequent history was probably roughly as follows:—At first the lake was connected to the northern lakes and rivers which now drain to the Nile. It was then isolated by the uplift of land immediately to its north, and was much reduced in volume during the arid interpluvial of the middle Pleistocene, with the result that the salts in its water became concentrated. During this period it is probable that the pseudo-marine types of Mollusca etc. were evolved. The second pluvial period, which followed aridity, refilled the lake and reduced the concentration of salts, but isolation from other waters may have continued till the formation of the Mufumbiro volcanoes in the late Pleistocene. These volcanoes dammed the upper reaches of the Ruchuru River, and caused ponding, with the result that Lake Kivu was formed and the drainage was reversed to a southerly direction, viâ the Ruzizi River, into Lake Tanganyika. This extra supply of water overflowed Lake Tanganyika, and an outlet was cut on the western shore, putting the lake into faunal communication with the Congo system.

Ecology.—Some evidence about the food and habitats of the species has been obtained from examinations of stomach contents, so that the fish can be divided into groups depending on their feeding habits, and a simple food-chain diagram has been constructed.

Systematic.—A key for the identification of the species is included, principally for use by naturalists who have not specialized in systematic ichthyology. Five new species are described, seven are redescribed, the species of Chrysichthys from Lake Tanganyika are completely revised, and the generic description of Phyllonemus is modified. In all cases where Christy's specimens are not typical the differences are noted in detail, in the hope that this may lead to a better understanding of the variation within the species and also of the production of endemic forms, whether varieties, species, or genera, under the influence of isolation.