S. F. Baird on North American Birds. 83 
lowing up the direction of the former river to the approaches of 
the Great Desert nearly on the meridian mentioned, proceeds 
northward, forced sometimes more or less westward, especially 
along the Platte, sometimes eastward. It crosses the Platte be- 
tween Forts Kearney and Laramie and intersects the Missouri 
between Fort Randall and Fort Pierre, perhaps near Fort Look- 
out, as it is between the first mentioned two points that in as- 
cending the river we find the change to take place in the orni- 
thology of the country. Soon after crossing the northern 
boundary of the United States and to the western side of Lake 
Winnipeg, the line rapidly inclines westward, especially beyond 
the Saskatchewan, and extends to the Rocky Mountains, includ- 
ing the valleys of Athabasca and Great Slave Lakes, and both 
sides of the Mackenzie River, north to the Arctic ocean, even 
crossing the Rocky Mountains to the Porcupine river and into 
Russian America at least to 145°, or beyond the forks of the 
Yukon, where Mr. Kennicott found many of the most character- 
istic summer land birds to be almost identical with those of 
Slave Lake, Lake Winnipeg, and Northern Canada. 
States, although its extent southward along the peninsula of 
Lower California is not well determined. ‘l'o the northwest it 
already stated, it extends along the eastern slope of the Cascade 
and Sierra Nevada mountains, and apparently along the east side 
of Lower California to Cape St. Lucas, at least the birds of the 
Cape, as will hereafter be explained, belong much more em- 
‘Summer may be 
