One final point that may not be apparent to contemporary readers.  We have a hard time understanding today why anyone would think a spherical earth implies habitation on the far side...after all, the entire backside could be endless ocean, right?  Or barren desert?  But to the early Medievalist, the earth was created by God, for the purpose of providing home to mankind.  It seemed incomprehensible to such men that God would create an entire "second world", below and unreachable from the first...and leave it wholly empty.  In their anthropocentric viewpoint, this was outrageous.  And this is much of the justification for the tacit linkage between "rotundity" and "inhabited antipodes" in many early MA sources.