hoe 
XXII HETEROMI 2k 
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. 
Fic. 378.—Columbia transmontana, natural size. (After Eigenmann.) 
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 epiotie, 
the post-temporal small and simple or replaced by a hgament ; 
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 
Giinther lead us to believe,’ 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. 8S. Rowntree, 
who has great experience in these matters, has kindly examined at my request a 
well-preserved example of Halosauropsis macrochir, and informs me that “ the air- 
