ICHTHYOLOGY. 
13 
These fourteen fresh water genera having the air-vessel enclosed in hone are divisible 
as follows:— 
1. —’Waters of plains— 
(«.)—Large rivers- No suckers on the chest:— Ailia, Ailiichthys, Sisor, JBagarius, 
Silundia, Eutropiichthys , Gag ala, 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:— Pseudeoheneis. 
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 hut 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 Nemacheili ) from Yarkand have the air-vessel 
free in the abdominal cavity, it stands to reason that heat or cold can scarcely he that which 
involves the necessity of this form of organization. 
It appears most probable that the air vessel being more or less enclosed in hone is for 
the purpose of developing some function specially required or to an abnormal extent, and that 
whatever this may he it is most necessary in a mountain torrent, hut unnecessary in a 
marine existence. 
We find in fishes that the air-vessel has twm 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. 
3) 
