Echinoconchus are quite large and occur is some abundance in the Keokuk Formation in my neck of the woods (Midwest USA). I believe the time frame on that Fm is late Osagean, but I may be off a Stage. Still, that should be similar in age to your stuff. Index Fossils of N America states that spines are present on both valves of Echinoconchus, but are seldom preserved.
Is the Big Snowy Group in the Chesterian Stage?
I for one have never found any Echinoconchus that have the spines preserved. Whatever genus/species yours turn out to be, they are flat out gorgeous!