Strophomena septae

Locally common? Most of those individuals were found in one layer this year. One of three official species of Strophomena from Decorah/Platteville Formation at least in Minnesota. #1 It resembles Strophomena filitexta into having an enclosed muscle scar. But it differs greatly from Strophomena filitexta into several ways - one of the most noticeable ways is into having a prominent central septae that run down the center. The other two Strophomena species lacks that prominent central septae that runs all the way down the shell. #2 is a false sulcus that runs down the outside of the shell. In vast majority of individuals that I have looked at, only one half have the false sulcus on the outside and often it is not that easy to find, being a subtle depression that runs down the middle of the shell on the outside. #3 On the outside of the shell the costae is more uniform compare to the other two Strophomena species.