240 
being apparently more useful for auditory than for hydrostatic 
functions, and as a general rule being smaller in fresh-water 
than in marine species. These fishes are evidently closely allied 
to the Cyprinide, and in one respect, the air-bladder, shows a 
somewhat similar modification. I have remarked how in the 
erovelling loach and some allied forms it is more or less 
enveloped in a bony capsule, and this is seen both in European 
and Asiatic forms, all of which, however, may be said to be 
ground feeders. The Siluwride of Asia likewise live the life of 
ground feeders, and the power of employing their air-bladder 
as a float appears to be subservient to that of hearing. In the 
marine forms it has thickened walls, and the parapophyses of 
the first vertebrae (ex. Arius subrostratus) form expanded plates, 
to the under surface of which this organ is attached. As we 
go inland, especially towards the Himalayas, this organ becomes 
more and more enveloped in bone until it is as we find it in the 
loaches, while, like the Cyprinide, a chain of ossicles passes 
forwards to the internal ear. In the marine forms the broad 
plate on its upper surface and bony stays to its partitions would 
appear to exist for the purpose of counteracting superincumbent 
pressure; while in the fresh-water forms this bony covering, 
being greatly increased, would seem to be due to some fresh- 
water physical cause, not to a tropical climate, as I have 
observed the same phenomenon is seen in European loaches. 
The number of fresh-water genera existing among the. Dipnoids 
of India and Burmah is twenty-five, out of which fourteen 
have the air-bladder more or less enclosed in bone; and, as 
all are ground feeders, one reason at least must be to prevent 
undue pressure on that organ when at great depths, and to 
preclude any abnormal interference with the function of hearing. 
The chain of auditory ossicles which connect the air-bladder 
with the internal ear is confined, so far as I am aware,* to the 
* Most probably this chain of ossicles will be found in some other 
fresh-water families, but which I have not yet had the opportunity of 
investigating. Since this paper was read I have discovered that auditory 
ossicles are present in the fresh-water family Gymmnarchide of the Nile: a 
fish which possesses a cellular air-bladder that appears to have a lung-like 
function. 
—_eooou ee ee 
