62 BULLETIN 188, UNITED STATES NATIONAL MUSEUM 



This fish seldom exceeds a length of 30 cm. in Thailand, but in other 

 countries it is reported up to 37 or 38 cm. 



Giinther (1861) had a specimen from "Pachebore," Thailand, ob 

 tained by Mouhot, and called it var. siamenms^ with the statement: 

 "Having only one specimen, we do not feel that we should be justi- 

 fied in describing it as a separate species." The differences from 

 typical aculeata^ namely, 13 instead of 18 to 20 dorsal spines, 58 instead 

 of 52 soft anal rays, and a pair of confluent ocelli at the base of the 

 caudal fin, seem to come within the normal variation in the species. 

 Hora's specimen from Bangkok had 15 dorsal spines, and Day gave 

 the dorsal spines as 16 to 20 in Indian examples. The black white- 

 edged ocelli along the base of the dorsal fin number 2 to 9; excep- 

 tionally there may be, in addition, one or two ocelli at the base of the 

 caudal fin and more rarely an ocellus at the posterior base of the anal 

 fin (Fowler, 1934a). 



The variation in the number and position of the ocelli is well il- 

 lustrated by fish in the Deignan collection from brooks tributary to 

 the Meong, an affluent of the Mekong, between Wieng Papao and 

 Chiengmai, Northern Thailand, July and August 1935. Of seven 

 specimens, three have 3 subdorsal and 1 caudal ocelli; two have 

 2 subdorsal and no caudal ocelli; one has 5 subdorsal and 1 caudal 

 ocelli; and one has 1 subdorsal and no caudal ocellus. The dorsal 

 spines in these specimens number 14 or 15. 



The generic name usually given to this species is Rhynchohdella^ 

 dating from Bloch (1801) , with orientalis Bloch as the type, that species 

 being a synonym of Ophidium aculeatwrn Bloch (1787) . In 1800, how- 

 ever, Lacepede had established the composite genus Macrognathus^ 

 which Cuvier and Valenciemies (1831, vol. 8) restricted to aculeatus, 

 and placed Lacepede's other species, armatus^ in Mastocembelus. 



This fish is well known to the Thai, and wherever found it is called 

 •pla lot^ a distinctive name. 



Genus MASTOCEMBELUS Scopoli 



Mastocembelus Scopoli, Introductio acl historiam naturalem, p. 458, 1777. (Type, 

 Mastacembelus unicolor (Kuhl and van Hasselt), Cuvier and Valenciennes.) 



The best taxonomic treatment of these fishes is by Boulenger (1912). 

 The characters on which he separated all the species known to him have 

 been used in the following key, into which have been incorporated four 

 recently described forms peculiar to Thailand. Several other Burmese 

 species may be looked for in Siamese waters of the western drainage. 



la. Dorsal and anal fins confluent with caudal. 



2a. Snout entirely scaly ; no preopercular spines ; dorsal rays XXVI to XXX, 

 60 to 70; anal rays III, 59 to 69; body brown, clouded with darker; 

 vertical fins with a yellow margin; a series of black spots at base of 

 dorsal fin maculatus 



