of certain Fishes of the Ganges. 41 



on the external surface of the base of the cranium, having mere, 

 lv a thin septum, generally of a membranous nature, interposed 

 between it and the principal ossiculum, or calcareous body of 

 the internal ear. 



In the Mystus Chetala this organ extends in length from the 

 first vertebra to near the extremity of the tail. Anteriorly, it 

 forms several chambers communicating with each other, and 

 behind is divided into two parallel cavities by a longitudinal 

 septum, which, like that of the Trichopodus Colisa, is partly 

 membranous, and partly composed of the spines that sustain the 

 anal fin. These spines are long, hollow, cylindrical bones, and 

 are connected by the internal lining coat of the bladder, reflect- 

 ed on both sides over their whole extent of surface. The la- 

 teral muscles forming the walls of the cavity are very thin, and 

 send off* from their lower part small slips of muscles that are in- 

 serted into the membrane between the spines, by which means 

 the septum can be expanded somewhat in the manner of a fan. 

 A free communication exists between the two sides of the organ, 

 through a number of foramina in the membrane connecting the 

 spines, and anteriorly between one of the chambers and the 

 oesophagus, by means of a round opening, wide enough to admit 

 the point of the little finger. From the anterior part of the 

 organ a canal proceeds upon the inferior surface of the cranium, 

 as far as the cavity containing the internal ear, opposite to 

 which it terminates in a blind extremity. Tt consists of a deep 

 furrow on the external surface of the base of the cranium, 

 covered below by an elongation of the external coat of the 

 bladder, (bearing some analogy in its structure to the Eusta- 

 chian tube of the higher orders of animals ;) and like the cor- 

 responding part in the Bola Pama, is separated from the prin- 

 cipal ossiculum of the internal ear, by a septum so thin and 

 delicate that the slightest touch with a pin is sufficient to rup- 

 ture it. (See Note A.) 



The air-bladder of the Clupanadon llisha is long, narrow, 

 and pointed at both extremities, as in the genus Clupea. 

 From its anterior extremity two very minute ducts arise, and 

 proceed along the inferior surface of the cervical vertebra? and 

 posterior part of the head, to terminate in two small air-bags, 

 situate one on each side of the cranium. These ducts, when 



