Vintage shell dress, confused

[CatherineMarieN]

Says: "Divide work for Back and Front": Mark off first 10 shells for back(place a marker on first and last shell for underarm) leave next 11 shells for front. Work as follows."

[Granny Square]

Sounds like you have worked the dress bottom up in the round, and are now needing to divide to make a slit for the armholes, and will start working the back only up to the neck, and then come back, reattach the yarn and work the front (it says leave 11 shells for front, implying 'leave for later').