434 
BIE. R. GURNEY ON THE CRUSTACEAN PLANKTON 
in the post-GIacial history of the region, penetrate up the St. Lawrence 
valley and flood the basins of Lake Ontario and of Lake Champlain. At 
this time there existed an eastern outlet from Lake Huron through the 
Ottawa valley. The marine deposits of this stage are found at a height of 
400-500 feet by Lake Champlain and at 600 feet near the east end of Lake 
Ontario. It would appear that we have here all the conditions necessary for 
the isolation and modification of these two " I'elict " species, but there are 
Table 7. 
Mysis 
relicta. 
Limnocalanus 
macritrus. 
Lalie Ontario* 
„ Michigan 
„ Superior 
+ 
+ 
+ 
+ 
+ 
+ 
+ 
+ 
+ 
+ 1 
+ 
+ 
+ 
+ 
+ 
+ 
„ St. Clair 
., Erie 
+ 
+ 
+ 
+ 
+ 
The Finger Lakes : — 
Cnnandaigua 
Cayuga 
Keulra 
Skaneateles 
two great difficulties in the waj-, namely the absence of Mysis relicta from 
Lakes Ontario and Chiimplain and the existence at or before tliat time of the 
Niagara Falls, which would be an effective bar to migration into the other 
lakes. On the other hand, the absence of a species from a lake at the 
present day does not necessarily prove that it has never existed there, and 
the occurrence of both species in the Finger Lakes, which form part of the 
drainage system of Ontario, seems to be gOod evidence that they have 
reached these lakes from Ontario. Their penetration into the other Great 
Lakes must, one must suppose^, have taken place by way of the Ottawa con- 
nectioHj and in any case we are forced to assume for L. macrurus a capacity 
for active migration up rivers which is denied by Ekman. 
* So far as I know, there is no puhlished record of the occurrence of Limnocalamis in 
Lake Ontario, but Dr. 0. Juday informs me that lie has found it in a plankton sample from 
this lake taken by Dr. N. A. Clement in October 1922. I am greatly indebted to Dr, Juday 
for this information. 
