THE ANIMALS OP 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 giUs, 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 tiU 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. All 

 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 speciaHzed breathing- 

 organs. Their ' discontinuous distribution ' over the 

 surface of the globe is another point of interest in 



