^TsqY''] proceedings of the national museum. 593 



Unio trigonus Lea. Fourteen valves. Its uortheiu limits are west- 

 ern New York, southern Michigan, and St. Peters, Minn. 



Unto solidus Lea. Eleven valves. Not hitherto reported outside the 

 Mississippi Basin. 



Unio elavus Lam? Five valves in bad conditioD, which, after the 

 most careful and exhaustive comi^arisou, 1 refer to this species. It is 

 confined to-day to the Mississippi area, reaching north into western 

 New "Y ork. 



Quite a number of specimens of the other fresh-water shells were 

 received in bad condition. These are Fleurocera elevatum Lea, 1\ sub- 

 ulare Lea, P. pallidum Leal and an undetermined species j Valvata 

 sincera Say, remarkably depressed, and Sphcvrium striatinum Lam. 

 All of these are now found living in Canada, except the first mentioned 

 species, which is, I believe, confined to the Mississippi area. 



The theory founded by Agassiz and elaborated by Dawson, Upham, 

 Gilbert, Tyrrell, and others, that during the glacial period the arcluean 

 region of Canada was elevated from 1,000 to 2,000 feet above its 

 present level, and that it was covered with an ice mantle from 3,000 

 to 6,000 feet thick, a mantle which in the eastern part of the United 

 States extended down to latitude 38° or 40°; that in the Champlain 

 period which followed there was a subsidence over this area, during 

 which great lakes were formed by the melting ice, whose northern 

 shores were the yet remaining wall of ice, and whose southern borders 

 were the land that sloped northward; and that they drained into the 

 Mississippi system, is most strongly confirmed by the evidence of these 

 fossil Unios, and by every fact of the distribution of the Naiades in this 

 general region to day. It is believed that the entire system of the 

 present Great Lakes was united, and that at one time it covered a con- 

 siderable part of lower Michigan, and extended well into Ohio, Indiana, 

 and Illinois. What has since become the lied Eiver of the Nortli, which 

 at that time was an arm of the great lake Agassiz, no doubt had its outlet 

 into the Upper Mississippi from the small Bois des Sioux Elver, which 

 rises in Lake Traverse, and from this connected with Big Stone Lake, 

 near by, the head of the Minnesota Biver.* The waters of the St. Law- 

 rence, dammed with ice, could only escape into the Mississippi system. 



It is quite probable that if the species of Naiades Avhich now are found 

 on the Atlantic slope inhabited any considerable part of the upper St. 

 Lawrence and northern drainage systems previous to the glacial 

 period, the great cajj of ice grinding over the country, together with the 

 rigorous climate, nearly or quite exterminated them in this area. As 

 evidence in this direction, the case of Margaritana margaritifera may 

 be cited. It is an oriental species, having its metropolis in northern 

 Europe and Asia, which has crossed over into North America in all 



* Geological and Natural History Survey of Canada, new series, Vol. vi, 1888-'89, 

 p. 5. E. 



Proc, N. :\r. 93 38 



