THE ANIMALS OF LAKES AND RIVERS 201 
has never been systematically sounded, but depths of 
1,200 to 2,100 feet have been recorded at various 
localities. The elevation is 2,624 feet above sea-level. 
The fish fauna show some curious features. In the 
first place the lake contains a species of Protopterus, 
one of the three living genera of dipnoi or lung-fishes. 
The two other living genera inhabit the one (Lepido- 
siren) the rivers of tropical America, and the other 
(Ceratodus) those of Queensland. As well as occurring 
in Lake Tanganyika, the species of Protopterus have 
a wide distribution in the lakes and rivers of the middle 
portion of the African continent. All the dipnoi 
possess, as we have seen, both lungs and gills, but the 
two sets of breathing-organs are not in use simul- 
taneously. The fish normally inhabit regions where a 
periodical dry season occurs, when the water in which 
they live either becomes very foul or dries up. The 
first condition is frequent in the natural habitat of the 
Queensland Ceratodus, the second in that of the two 
other genera. At this period, therefore, Lepidosiren and 
Protopterus make themselves nests of mud, in which 
they lie dormant, breathing by their lungs till the 
water returns. In the case of Ceratodus the lungs are 
used when the water is too foul for the gills to be of 
any use, but the animal does not make mud nests. AIL 
three genera show certain very primitive characters 
in combination with this specialized mode of breathing. 
Their ancestors were certainly marine, many fossil 
forms having been found. Thus the dipnoi must be 
looked upon as the fresh-water survivors of a highly 
primitive group of marine fishes, owing their per- 
sistence to the acquisition of specialized breathing- 
organs. ‘Their ‘ discontinuous distribution’ over the 
surface of the globe is another point of interest in 
