370 BULLETIN 188, UNITED STATES NATIONAL MUSEUM 



the whole dental mass forming a crescent. Fowler shows two rounded 

 patches of vomerine teeth and no palatines. As the specimens in the 

 two collections are otherwise in agreement, it may be assumed that in 

 this form, as in Pangasius^ the dentition is subject to variation. 



Genus HELICOPHAGUS Bleeker 



HeUcopharjus Bleeker (174), Act. Soc. Indo-Neerl. (Sumatra), vol. 3, p. 45, 

 1858. (Type, Helicophagus typus Bleeker.) 



The helicophagids are very closely related to the pangasiids. Both 

 have 4 barbels, a maxillary pair and a mandihulary pair (which 

 latter may be rudimentary or altogether absent in large examples of 

 some species) . The important point of difference is that whereas the 

 pangasiids have both vomerine and palatine teeth, the helicophagids 

 always lack palatine teeth. The front border of the snout is pierced 

 by the anterior nostrils, which are directed forward or upward, while 

 the posterior nostrils are midway between the anterior ones and the 

 eyes. 



The genus Helicophagus is well named. Of the various specimens 

 examined by me at different time and places in Thailand, all had entire 

 shells of small univalve mollusks in their stomachs. The two known 

 species from Thailand may be differentiated as follows: 



Irt. Anal rays 30; maxillary barbel not extending beyond preopercle; mandibu- 

 lary barbel not extending beyond eye ; vomerine teeth in 2 straight widely 

 separated bands hypophthalmus 



lb. Anal rays 37 to 38; maxillary barbel extending beyond base of pectoral fin 

 or not so far; mandibulary barbel reaching base of pectoral fin or not so 

 far ; vomerine teeth in 2 small oblique widely separated patches. 



waandersii 



HELICOPHAGUS HYPOPHTHALMUS Sauvage 



Helicophagus hypophthalmus Sauvage, 1878b, p. 235 (Laos, Indo-China) ; 1881, 

 p. 170, pi. 7, fig. 1 (Laos, Mekong; Grand Lakes of Cambodia, Indo-China). 



Pangasms hypophthalmus Hoea, 1923b, p. 166 (Bangkok). — Fowlee, 1934a, p. 

 88 (Bangkok). 



Although both Hora (1923b) and Fowler (1934a) placed Sauvage's 

 HelicopJiagus hypophthalmus in the genus Pangasius, there is noth- 

 ing in Sauvage's description to indicate that his species, of which he 

 had specimens from Laos, Grand Lakes of Cambodia, and the Mekong, 

 is not a Helicophagus. A character adopted by Bleeker and by Weber 

 and de Beaufort to separate Helicophagus from Pangasius, namely, 

 the presence in the latter of palatine teeth, clearly excludes H. hy- 

 pophthalmus from Pangasius. Sauvage's account is very definite on 

 this point, and may properly be read in conjunction with his adjoin- 

 ing account of new species of Pangasius^ in which he distinctly de- 



