324 DISTRIBUTION AND ZTIOLOGY OF THE CRAYFISHES. 
the stock from which they have been evolved. Even 
under existing geographical conditions, an affluent of the 
Mississippi, the St. Peter’s river, communicates directly, 
in rainy weather, with the Red river, which flows into 
Lake Winnipeg, the southernmost of the long series of 
intercommunicating lakes and streams, which occupy the 
low and flat water-parting between the southern and the 
northern watersheds of the North American Continent. 
But the northernmost of these, the Great Slave Lake, 
empties itself by the Mackenzie river into the Arctic 
Ocean, and thus provides a route by which crayfishes 
might spread from the north over all parts of North 
America east of the Rocky Mountains. 
The so-called Rocky Mountain range is, in reality, an 
immense table-land, the edges of which are fringed by two 
principal lines of mountainous elevations. The table- 
land itself occupies the place of a great north and south 
depression which, in the cretaceous epoch, was occupied by 
the sea and probably communicated with the ocean at its 
northern, as well as at its southern end. During and 
since this epoch it became gradually filled up, and it ¢ 
now contains an immense thickness of deposits of all 
ages from’ the cretaceous to the pliocene—the earlier 
marine, the later more and more completely freshwater. 
During the tertiary epoch, various portions of this area 
have been occupied by vast lakes, the more northern of 
which doubtless had outlets into the Northern sea. That 
erayfish existed in the vicinity of the Rocky Mountains 
