FRESH-WATER PEARL MUSSELS 
53 
chusetts was raised to a much higher level than it is now, 
and that it has gradually sunk to its present position. If this 
had been the case, the rivers on the western side of this 
highland would have drained westward and would have 
formed part of the Mississippi drainage, while the eastern 
streams would have emptied their waters straight into the 
Atlantic. The latter ought therefore to show distinct signs 
in the composition of their fauna of having been isolated 
from the Mississippi drainage system. Dr. Simpson de¬ 
scribes the fresh-water pearl-mussels of the Mississippi 
drainage system as large, ponderous and ornamented with 
beautiful and odd patterns of colour and sculpture. The 
waters that drain into the Atlantic on the other hand are 
inhabited by a totally different set of mussels. They are 
moderate in size, frail and not remarkble for either colour 
or sculpture. Only very few of the eastern species extend 
westward to the head waters of the St. Lawrence.* 
In the interesting report on the Pleistocene fauna and flora 
already cited, there is a list of twelve species of fresh-water 
mussels that have been observed in the Don valley deposits. 
None of them belong to extinct species, though it is a signifi¬ 
cant fact that nearly one half of them no longer inhabit the 
St. Lawrence drainage, being now confined to the Mississippi 
and its tributaries.f 
This certainly implies that the Mississippi was more inti¬ 
mately connected with the upper St. Lawrence system in 
early Glacial and probably in pre-Glaeial times than it is now, 
for it is not unjustifiable to assume that these southern forms 
found their way northward before the commencement of the 
Glacial Epoch. It is possible that those southern species, 
found in the Don deposits, which still have a wide range in 
Canada, may have survived the Glacial Epoch in that country. 
Dr. Simpson J pointed out that almost all the Mississippi 
valley species of mussels now living in St. Lawrence drain¬ 
age area have become more stunted and more simple in 
outline, while their characteristic sculpture and colours are 
* Simpson, C. T., “ On some Fossil Unios.” pp. 591 — 592. 
f Dawson, J. W., D. P. Penhallow, &c., “Canadian Pleistocene Flora 
and Fauna,” p. 331. 
X Simpson, C. T., “On Mississippi Valley Unionidae,” p. 384. 
