This is true, but this was only the shield the legions used, and then only the earlier Imperial ones. The auxiliaries used round or oval shields, depending on their national traditions, and this eventually spread to the legions themselves. Presumably it was the fact that the Vandals, Huns and other hordes defeated the Western Empire that made this design unpopular after the Dark Ages (qv).
I think the Eastern (Byzantine) Empire continued to use the oval/round versions, but I can't quote chapter or verse.
--Simoncursitor 09:00, 6 Apr 2005 (CDT)