Two versions of the opening argument:
1798:
THE RIME OF THE ANCYENT
MARINERE, IN SEVEN PARTS
ARGUMENT.
How a Ship having passed the Line was driven by Storms to the cold
Country towards the South Pole; and how
from thence she made her course to the tropical Latitude of the
Great Pacific Ocean; and of the strange things
that befell; and in what manner the Ancyent Marinere came back
to his own Country.
Becomes in 1800:
The Ancient Mariner. A Poet's Reverie.
How a Ship, having first sailed to the Equator, was driven
by Storms, to the cold Country towards the South Pole;
how the Ancient Mariner cruelly, and in contempt of the
laws of hospitality, killed a Sea-bird; and how he was followed
by many and strange Judgements; and in what manne he came
back to his own Country.
Here is a link to more information & the poem: http://etext.lib.virginia.edu/stc/Coleridge/stc.html
ps. I do not see the epigraph in our edition in either the 1798 or the 1800. I suspect it was added in 1817, but have not verified this.