Lakeside Shells at Buffalo Pound

I lived beside the ocean for so long as a child that when I see a shell, I automatically think, 'seashell.' But there are land snails, too, and molluscs that live in lakes.

On Saturday, we drove out from Regina to Buffalo Pound Provincial Park to enjoy the late summer weather (33+C). These pictures were taken at the edge of Buffalo Pound Lake, where these (not sea!) shells lay in a line of debris washed along the shore by waves from speedboats and the wind.

What are these? Prairie lake shells, washed ashore?  (or land shells?
Location: Buffalo Pound Lake, Saskatchewan.
Photo Date: September 24, 2011. 

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