PROFESSORS T. W. BRIDGE AND A. 0, HADDON 
R14 
abnormales. Further investigation may prove that a considerable number of Cypri- 
nidse besides the Cobitidihae possess reduced and bone-encapsuled air-bladders, more 
especially those hilly or Alpine genera, such as Gymnocypris, Oreinus, Schizothorax, 
Ptychoharhus, Schizopygopsis, Diptychus, and others, that certainly live under con- 
ditions in every respect very similar to those which in so many Siluridae are 
associated with a degenerate bladder. The small East Indian Cyprinidae which 
are included in the sub-group Homalopterina are usually described as entirely with- 
out an air-bladder. Tliey are said to be exclusively ground forms, with the ventral 
surface of the body flattened for adhesion to stones at the bottom of the rapid hill 
and mountain streams to which they are restricted. The habits of these Fishes 
would certainly lead us to expect that the air-bladder would be rudimentary and 
enclosed within bony capsules, but its complete suppression is unique so far as the 
Ostariophysese are concerned. But in view of the fact that the very existence of an 
air-bladder has been denied in the case of several Siluroids which are now known to 
possess at least a vestige of that organ, the statement that it is completely absent 
in the Homalopterinae cannot be accepted without further confirmation. 
VI. — Concluding Kemarks. 
The many and varied structural modifications met with in the air-bladder of the 
Siluridie are not surprising in view of the exceptionally diversified conditions under 
wdiich the different species and genera live. Their geographical distribution is almost 
co-extensive with the tropical and subtropical regions of the Earth’s surface, and both 
the habitat and habits of the different species are remarkably varied. Some Alpine 
species are restricted to mountain streams often at an altitude of several thousands of 
feet above the sea level, and sometimes reaching a height of 15,000 feet, as is the 
case with certain Siluroids of the Andes of South America ; other species are charac- 
teristic of the streams of the hilly country of a lower level ; while other species again 
are equally characteristic of the larger rivers and lakes of the plains ; and a limited 
few are marine, extending even to some of the Oceanic islands of the Pacific. The 
climatic variations to which the Siluridae are subject are almost as diversified. 
Though mainly tropical or subtropical, certain genera are exposed to an almost Arctic 
climate, at least for a portion of the year, the result either of the altitude at which 
they live, or of their extension to the confines of the north temperate regions. The 
physical conditions under which many Siluridse are capable of living are almost as 
varied as their geographical distribution or climatic range. Many of them are ground- 
feeders ; some of them have a remarkable power of living out of water ; others are 
liable through prolonged drought and the consequent drying up of rivers or lakes to 
become buried in the mud for longer or shorter periods without subsequent injury; 
and others again are said to be capable of migration upon land from river to river. 
Darwin* has pointed out, in referring to the conditions favourable to variation in 
* ‘ The Origin of Species,’ fith ed., pp. 42 to 43. 
