584 Bulletin American Museum of Natural History [Vol. LIII 
half is probably over 1,100 m. deep over much of its area and soundings 
here have reached to 1,277 m. below the surface. The southern basin 
shows even greater depths, in one place as much as 1,485 m. The waters 
are of a dark green color and slightly brackish, though drinkable. They 
contain a relatively small amount of calcium salts, but are unusually rich 
in salts of magnesium.! The diurnal temperature of the surface water 
oscillates between 25° and 29° C.; it is slightly lower beneath the surface 
and remains fairly constant at about 23° C. between 400 m. and the 
bottom. The lake is subject to violent tempests and is usually quieter 
during the night than the day. These heavy movements of the surface 
water not only influence mechanically the distribution and structure of 
plants and animals, but they also contribute actively in aérating the 
water so that they are partly responsible for the abundance of life at 
greater depths.?. L. Stappers found the transparency, as measured with 
a white disk, to vary from 2 m. to 8 m. 
Tanganyika thus possesses many of the physical and perhaps also 
some of the mechanical features of the ocean. If such marine-like condi- 
tions lasted sufficiently long in past geological periods, while the lake 
remained isolated from other fresh-water bodies, they may go far to 
explain its remarkable forms of life. The flora, as far as higher aquatic 
plants is concerned, shows nothing distinctive; but the Alge have proved 
to be a strange and interesting collection, a considerable number being 
peculiar to the lake, while several show undoubted marine affinities. 
The fauna, however, is in every way unique. No other assemblage of 
animals has roused so much discussion among biologists and it will be 
necessary to consider it somewhat in detail before attempting to draw 
any inferences from it. | 
Considered as a whole, the animals of Tanganyika fall into two 
groups, which are especially well marked among the mollusks. Some 
forms do not differ conspicuously in general appearance from the usual 
fresh-water types and belong to genera or even species that occur in 
other African lakes or rivers. Others, however,—and they are the 
largest number, as well in species as in individuals—are totally unlike 
the members of the neighboring fluviatile faunz and differ from all 
Sp a ee ee ee eee et ee 
'These are evidently derived from Lake Kivu, where they are present in large quantities. 
es "Interesting physical data concerning Lake Tanganyika are contained in a series of papers by L. 
appers: 
1913. ‘Note sur les variations saisonniéres de l’embouchure de la Lobozi, affluent dela Tanganika.’ 
Revue Congolaise, IV, pp. 100-110, Pls. vi—-x. (Pl. x illustrates the action of the surf on the shore). 
1914. ‘Exploration scientifique du Dr. L. Stappers aux lacs Moéro et Tanganika. I. Recherches 
bathymétriques sur les lacs Moéro et Tanganika.’ Ann. Biol. Lacustre, VII, pp. 83-114, Pls. u-1v, map. 
913. ‘Mission Stappers, 1911-1918. Répertoire général des échantillons d’histoire naturelle 
recueillis.”. (Brussels), xii-+ [67] pp. 
