ICHTHYOLOGY. 
13 
These fourteen fresh water genera having the air-vessel enclosed in hone are divisible 
follows : — 
1. — Waters of plains — 
(«.) — Large rivers- No suckers on the chest : — Ailia, Ailiichthys, Sisor, Bagarius, 
Silundia, Eutropiichthys , Gagata, Nangra. 
(g.)— Large rivers : descending to the sea. An accessory air-breathing apparatus 
Clarias. 
(7.) — 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 hone decrease in number the further 
' vve 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 for ms . 
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 be that which 
Evolves the necessity of this form of organization. 
It appears most probable that the air vessel being more or less enclosed in bone is for 
fhe 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 
0r 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 
°pening 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 
mating seems to entirely supersede that for flotation, for being more or less enclosed in bone 
Contraction and expansion w r ould be impeded. These bones or auditory ossicles lead to the 
mternai 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 
C yprinincs of this collection have all small scales, or are more or less destitute of any ; 
;; h ilst the Loaches of Yarkand and Tibet have none at all; neither have those recorded fiom 
10 Oxns or the Jaxartes. ... 
There is one characteristic of the hill Loaches which seems almost invariable . t c 
Pectoral fins are stiff at their bases, as if employed for adhesive purposes. I have observed 
tbe outer ray in some of the Loaches of the plains forming a distinct bony ray with an 
mdargod and flattened outer extremity : but this is used for the purpose of assisting them to 
into the sand, in which they will bury themselves with great rapidity on the approach of 
danger. 
D 
