1830-3 
of certain Fishes of the Ganges. 
171 
by the spines that support the anal fin, into two cavities : strong tendinous fibres 
run obliquely between these spines, and fill up the space between them, leaving 
merely a few irregular openings of communication between the two sides above 
and below. 
In the Trtchopodus Colisa it differs from that of the former, in having a membra- 
nous septum in its abdominal portion ; thus dividing the organ into two compart- 
ments, from one extremity of it to the other. 
The air-bladder varies considerably as to figure, size, and internal structure, 
among the species of the genera Pimelodus and Silurus. 
In the P. Aor it usually consists of two portions of the figure of a heart, placed 
in a line, and united at their apices. 
It is extremely small in the P. Silondia in proportion to the bulk of the fish. 
It lies close to the anterior vertebrae, is of an oval figure, and is divided into two 
cavities, each of which, in a fish weighing eight pounds, is not larger than a hazel 
nut. 
The P. Pangasius has it composed of four or more portions, extending in a 
line from opposite the pectoral fins, to near the end of the tail. The first is 
generally oval, the second pyramidal, and the two last, which run between the 
caudal portions of the lateral muscles, approach to a cylindrical shape. The nu- 
merous septa on its internal surface descend from above downwards in the first 
portion ; in the second, they run in a transverse direction ; and in the posterior 
ones, form a number of irregular cells. 
In the Silurus Boalis it is of the figure of a heart, divided, internally, by a longi- 
tudinal septum into two cavities, which have a free communication with each other 
through a semilunar opening at the anterior part of the septum 1 . 
It communicates in all of these species with the alimentary canal, by a ductus 
pneumaticus , extending from its lower surface to the esophagus. 
The air-bladder, besides possessing the locomotive function generally ascribed to 
it, appears, from the connection that exists between it and a set of small bones, 
analogous to the ossicula anditus, to exert some influence on the sense of hearing. 
These bones correspond to the malleus , incus , and stapes of Mammalia, and are 
present, according to Professor Weber, by whom they were discovered, in all the 
osseous fishes in the vicinity of the anterior or cervical vertebra 2 . They exist in 
the different species of the genera Silurus, Pimelodus, and Cyprinus, I have had an 
opportunity of examining ; but are apparently wanting in the genera Ophiocephalus, 
Coins, and Trichopodus : and also in the Bo la Puma, Mystas Chetala, and Clupan- 
adon Ilisha, in which species their place is supplied by a direct connection between 
the two organs. The bone corresponding to the malleus is considerably larger 
than either of the other two, and is uniformly situate at the anterior exfremity of 
the air-bladder, extending along the side of the spine to opposite the first vertebra. 
In the Silurus Boalis its anterior half is long, flat, and of a triangular shape, ter- 
minating in a blunt point, while the posterior portion is crescentic, having below, 
a scabrous surface to which the external coat of the air-bladder is firmly attached. 
By means of a short styloid process, projecting from the side of the latter portion, 
and received into a pit on the body of one of the vertebrae, this bone enjoys a 
limited degree of lateral motion, by which its apex can be made to approach or 
recede from the spine. The incus , which is of a short cylindrical shape, is placed 
between the apex of the malleus, and the hone corresponding to the stapes, and 
at a right angle with the former : and is attached to each of these bones by a short 
round thick tendon, in the centre of which it presents the appearance of being im- 
bedded. The stapes may be described as consisting of two parts : one is a thin 
hollow portion, of an oval figure, somewhat resembling the bowl of a spoon, having 
the tendon of the incus attached to its convex side ; the other consists of a small 
round knob, with a minute spicula projecting from it, and is joined by a neck to 
the small end of the first portion. It is situate in a large foramen in the first 
vertebra, and forms one of the sides of a small chamber communicating with the 
cavity of the cranium. This chamber lies immediately behind the cavity contain- 
ing the sac and ossicula , or calcareous bodies of the internal ear ; and within the 
canal of the first vertebra, its roof being composed of a strong tendinous mem- 
brane, that supports tlie medulla spinalis above. The oval portion of the stapes , 
with its concave side presented towards the interior of the chamber, is loosely 
connected by a membrane, to the edge of the foramen, and admits of being project- 
ed into it, to such an extent as almost to touch its fellow of the opposite side, when 
1 This cavity generally contains a number of worms of the G. Fasciola. 
* Blumenbach’s Comparative Anatomy, p. 285. 
