HETEROMI 621 



may be regarded as highly specialised members, having evolved 

 in the direction of the Acanthopterygii. 



Only two genera are known, each with a single species : 

 Percopsis, from the rivers and streams of Canada and the north- 

 eastern United States, and Columbia, more recently discovered 

 in the sandy or weedy lagoons along the Columbia River. These 

 Fishes are of small size, not exceeding 6 inches in length. 

 Their eggs are unusually large. 



FIG. 378. Columbia transmontana, natural size. (After Eigenmaim.) 



Sub-Order 6. Heteromi. 



Air-bladder without open duct. Opercle well developed ; 

 parietal bones separating the frontals from the supraoccipital. 

 Pectoral arch suspended from the supraoccipital or the epiotic, 

 the post-temporal small and simple or replaced by a ligament ; 

 no mesocoracoid. Ventral fins abdominal, if present. 



The Halosauridae and Notacanthidae are deep-sea Fishes of 

 obscure affinities. In the abdominal position of the many- 

 rayed ventral fins and in the absence of the mesocoracoid arch 

 they agree with the Haplomi ; but if, as the investigations of 

 Grunther lead us to believe, 1 there is really no open communica- 

 tion between the air-bladder and the digestive tract, they 



1 Vaillant was inclined to take a different view, but with considerable diffidence, 

 owing to his inability actually to trace an open duct. I believe Giinther to be 

 right on this point, as well as in his account of the suspension of the pectoral arch 

 in Notacanthus, which I have been able to verify. Besides, Mr. W. S. Rowntree, 

 who has great experience in these matters, has kindly examined at my request a 

 well-preserved example of Halosauropsis niacrochir, and informs me that "the air- 



