EPICONTINENTAL SEA OF JURASSIC AGE 2609 
separating these two provinces is placed in the vicinity of Van- 
couver Island. The faunal interrelations of these two provinces 
are as follows: Of seventy-eight genera occurring in the two 
provinces nine are common to both ; of one hundred and four 
species six are common to both; andof ten circumpolar species 
which have reached Vancouver Island and Puget Sound only 
four occur in California, and but one in Lower California. ' 
From these conditions it will be seen that communication 
between the two provinces is almost, if not quite, as thoroughly 
prohibited now as it was during Jurassic times. The question 
which now arises is what restricts communication between the two 
provinces at present? It cannot be said to be due to climate 
alone, for why in that case should the circumpolar species be 
found so far south? And why should they all be found in Puget 
Sound and not be found farther south? This seems to be an 
exception to the general rule that the climatic provinces of the 
present time are connected by transition zones. For the line of 
demarcation is moderately sharp. 
Aside from the matter of climate there are two physio- 
graphic conditions which may be operative. The first of these 
lies in the extreme narrowness of the sumerged shelf lying to 
the north and west of Puget Sound. This shelf teeming with 
organisms already well established offers small inducement to 
migratory forms. And only the more hardy forms would be 
likely to survive the struggle for existence under such circum- 
stances as are here postulated. Thus the change of species 
from one province to the other is necessarily slow. 
There are good reasons for believing that throughout the 
Mesozoic era these topographic conditions of the Puget Sound 
region were much as they are at present. During the Horse- 
town epoch the Pacific shoreline, although it lay a considerable 
distance east of the present shoreline in California and Oregon, 
very closely approximated it in the Puget Sound area. The 
Chico also had a very restricted epicontinental area at that 
point as the Chico shoreline extended only to the eastern coast 
of Puget Sound. In Californiaand Oregon, however, its eastward 
