FRESH-WATER PEARL MUSSELS 58 



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-Glacial 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 $ pointed out that almost all the Mississippi 

 valley species of mussels now living in St. Lawrence drain- 

 age area Jiave become more stunted and more simple in 

 outline, while their characteristic sculpture and colours are 



* Simpson, C. T., " On some Fossil Unios." pp. 591592. 

 t Dawson, J. W., D. P. Penhallow, &c., "Canadian Pleistocene Flora 

 and Fauna," p. 331. 



| Simpson, C. T., "On Mississippi Valley Unionidae," p. 384. 



