in the Fish Gallery of the Indian Museum. 87 



Subclass DIPNOI [Cases 23-25]. 



From the map in this Case, showing the Geographi- 

 cal Distribution of this small subclass of Fishes it will 

 be seen that none occur in Asia and none north of the 

 tropic-;. As a matter of fact, they are now restricted to 

 the fresh waters of tropical and subtropical latitudes in 

 South America, Africa, and Australia. 



They differ from all other fishes, and approach Amphi- 

 bia, in having (1) an air-bladder that functions as a true 

 lung, and (2) an incipient pulmonary circulation. They 

 are further remarkable among fishes in the form of their 

 paired fins, which have a segmented axis foreshadowing 

 the segmented limbs of higher vertebrates. 



The skeleton is partly cartilaginous and partly bony, 

 and, as in the Sharks, the intestine has a spiral valve 

 and opens into a cloaca. 



The gills resemble those of Teleosteans and Ganoids 

 in lying free within a gill-chamber on either side of the 

 head. The gill-chamber has a gill-cover, though a 

 rudimentary one — and opens to the exterior by a single 

 gill-opening. 



The body is covered with cyloid scales. 



By means of their lung the Dipnoi can live in water 

 that is too impure for branchial respiration. 



The subclass is represented in this Gallery by a stuffed 

 specimen, a dissected pectoral fin, and a skull, of Cerato- 

 dus. In the skull notice the curious compound teeth. 

 Similar teeth have been found, fossil, in Secondary strata 

 in India and Europe. 



The Dipnoi are generally regarded as a divergent 

 branch of the stock from which the Ganoids descended.. 

 They are also connected, through the ancestors of the- 

 Chimseras, with the Chondropterygian stock. 



