Originally Posted by brutal_descent
They're most likely just off-dates from the Mastodon tour rather than tour dates.
No, it's a tour with Priestess immediately following Roadburn. It's why Priestess are playing a day early at Roadburn.
As for the NA off dates, you're missing:
18 - Pittsburgh, PA - Rex Theater (opening for DEP)
30 - Charleston, SC - Jimbo's Rock Lounge