FRESH-WATER FISHES OF SIAM, OR THAILAND 393 



This fish may be readily recognized by its long, compressed body; 

 small, pointed head ; pair of maxillary barbels extending to the caudal 

 fin; very slender nondenticulated dorsal spine, its length twice the 

 depth of the body ; very long, high adipose fin arising closely behind 

 the dorsal; deeply forked caudal fin, with the upper lobe always the 

 longer and often filamentous. 



Fowler (1934b) described Prajadhipokia rex as a new genus and 

 new species based on a specimen 17 cm. long from Bangkok. The next 

 year he pronounced P. rex a synonym of Heterohagrus hocourti. In 

 1937, however, having had before hira a specimen of H. hocourti 18.9 

 cm. long from the Nan River at Pitsanulok, which he described and 

 figured, he reconsidered P. rex as valid, owing to assumed differences 

 in such characters as gill rakers, teeth, coloration. The present writer 

 is unable to recognize as of generic or even specific value the minor 

 differences between specimens of H. hocourti and the description and 

 figures of P. rex, and believes they represent only individual variation. 

 Thus, Fowler records the gill rakers in P. rex as 4+9, slender, lanceo- 

 late, 0.75 of gill filaments, and in H. hocourti as 4+12, lanceolate, 0.8 

 of gill filaments. This difference of 3 gill rakers on the long arm of 

 the first gill arch cannot be regarded as important, and 3 specimens 

 taken at random from a lot of E. hocourti from the Lopburi River have 

 the gill rakers on one side 4+7, 4+10, and 4+11. 



The fish is well known to the fishermen and is given the name pla 

 kayeng with various qualifying adjectives. Thus, at Paknampo it is 

 called pla kayeng mu (mu, pig) in allusion to the shape of the muzzle, 

 at Lopburi and Korat it is known as pla kayeng tong or dong (tong, 

 banner) in allusion to the high dorsal fin, and on the Menam Sak the 

 long translucent adipose fin has given rise to the name pla kayeng hai 

 khao {hai khao, rice leaf) . 



Family SISORIDAE 



The Thailand representatives of this family fall into four genera of 

 widely different aspect which may be recognized readily by the follow- 

 ing characters : 



la. No thoracic adhesive apparatus. 



2a. Gill openings wide, extending on ventral surface ; dorsal fin with a strong 



osseous spine; adipose fin short, its base about equal to that of anal; 



caudal fin deeply forked ; pectoral and ventral fins not modified to act as 



an adhesive apparatus. 



3o. Head depressed, its upper surface naked and bony; eyes superior, mouth 



crescentic ; bases of mandibular barbels in a strongly curved line ; gill 



membranes united, free from isthmus. Size large, in rivers__ Bagarius 



3&. Head compressed, its upper surface covered with thin skin ; eyes lateral ; 



mouth straight, transverse ; bases of mandibular barbels in a straight 



transverse line; gill membranes joined to narrow isthmus. Size 



medium or small, in mountain streams Gagata 



