Fitzroy was also used by the children of
Charles II,
Henry I and
John
The Fitzcharles who were also the children of
Charles II and the Fitzclarences who were the children of
William IV (from when he was the
Duke of
Clarence.
(And to the OP: FitzWilliam is the name of an earldom (real life, not just in P&P) according to Wikipedia:
http://en.wikipedia.org/wiki/Earl_FitzWilliam, though being Wikipedia any information has to be taken with some caution).
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Louise x
~*~Top Names~*~
Charles Frederick & Isabel Rosemary