The Isinglass of Commerce. 245 



genus are peculiar, in having filamentous prolongations at 

 the base of each pectoral fin ; these are remarkably elon- 

 gated in Polynemus paradiseus, known in Bengal as the 

 Tupsi mutchi or mango-fish. By correctly ascertaining 

 the number of these prolongations in a specimen, a decision 

 may be arrived at whether the species possesses or is des- 

 titute of an air-vessel ; or, in short, if isinglass can or cannot 

 be obtained from it. 



But of seven species at present recognized in the seas and 

 estuaries of the coasts of India, merely two are useful for 

 this manufacture, and they are the only ones which have 

 five pectoral appendages. Consequently, unless a large 

 Polynemus possesses five of these filamentous appendages 

 at the base of its pectoral fin, it will be useless looking for 

 its air-vessel ; it has none, and isinglass cannot be pre- 

 pared from it. When dried, the air-vessel is tongue-shaped, 

 as are also those of others of the Acanthopterygians whose 

 air-vessels are loose in the cavity of the abdomen, have no 

 communicating duct leading to the pharynx, and are not 

 attached to the vertebrae. If we examine the air-vessels of 

 the Siluroid or scaleless cat-fishes, which are used for isin- 

 glass, we find them entirely different. They are like short 

 rounded bags with an open mouth, this latter being where 

 they have been torn away from their adhesions to the 

 vertebrae. The fishes which furnish these descriptions of 

 air-vessels are mostly found in muddy waters, estuaries, 

 and the mouths of rivers, but do not thrive where the sea is 

 clear. The Rita ritoides, C. and V., or Pimelodiis rita, H. B., 

 which attains a great size, and is found far up rivers, is 

 said to afford this substance in large quantities. 



Dr. McClelland discovered, about the year 1839, that 

 the Chinese had been importing isinglass from India in 

 enormous quantities, and from immemorial ages, and an 



