ICHTHYOLOGY. 13 



These fourteen fresh water genera having the air-vessel enclosed in bone are divisible 

 as follows : 



1. Waters of plains 



(a.) Large rivers. No suckers on the chest : Ailia, Ailiichthys, Sisor, Bagarim, 



Silundia, Eutropiichthys, Gagata, Nangra. 

 (g.) Large rivers : descending to the sea. An accessory air-breathing apparatus : 



Clarias. 

 (y.) Smaller rivers, tanks, &c. An accessory air-breathing sac : Saccobranchus. 



2. "Waters of the plains or hills 



No sucker on chest : Amblyceps. 

 Sucker on chest: Glyptosternum. 



3. Waters of hills- 

 Sucker on chest : Pseudecheneis, 

 Chest adhesive : Exostoma. 



As we find genera with the air-vessel enclosed in bone decrease in number the further 

 we are from Hindustan Proper, it is but natural to conclude that the necessity for this bony 

 capsule is greater in India than in other tropical countries, and also that it is only useful for 

 freshwater forms. 



When we see that all fishes (except the NemacJieili) from Yarkand have the air-vessel 

 free in the abdominal cavity, it stands to reason that heat or cold can scarcely be that which 

 involves the necessity of this form of organization. 



It appears most probable that the air vessel being more or less enclosed in bone is for 

 the purpose of developing some function specially required or to an abnormal extent, and that 

 whatever this may be it is most necessary in a mountain torrent, but unnecessary in a 

 marine existence. 



We find in fishes that the air-vessel has two distinct functions 



(1). In the Acanthopterygii, where it is free in the abdominal cavity, its use is more 

 or less a mechanical one, and by contracting or expanding the fish is enabled to maintain 

 itself at a desired level. 



(2). In the Physostomi we find a very different formation, as in all there is a duct 

 opening from the air-vessel into the upper portion of the alimentary canal. In some of 

 these fishes the mechanical function appears to be alone served by it. In others, that of 

 hearing seems to entirely supersede that for flotation, for being more or less enclosed in bone 

 contraction and expansion would be impeded. These bones or auditory ossicles lead to the 

 internal ear, and it is evident that in some way the air vessel serves for auditory purposes 

 to an extent for which we, at present, are hardly in a position to account. 



It is remarkable that Siluroid forms do not appear to thrive in cold climates. The 

 Cyprinince of this collection have all small scales, or are more or less destitute of any ; 

 whilst the Loaches of Yarkand and Tibet have none at all ; neither have those recorded from 

 the Oxus or the Jaxartes. 



There is one characteristic of the hill Loaches which seems almost invariable : the 

 pectoral fins are stiff at their bases, as if employed for adhesive purposes. I have observed 

 the outer ray in some of the Loaches of the plains forming a distinct bony ray with an 

 enlarged and flattened outer extremity : but this is used for the purpose of assisting them to 

 dig into the sand, in which they will bury themselves with great rapidity on the approach of 

 danger. 



