tar won't extract a dir

[Glenn McCalley]

Hello all, never had tar refuse to extract a directory from an archive 
before.

tar archive is:
htdocs.1201.tar.gz

tar tzf htdocs.1201.tar.gz | grep standrewsglenwood
...shows a list of files contained within the archived dir:
htdocs/standrewsglenwood/<many whatevers>

yet the command:
tar xzf htdocs.1201.tar.gz  htdocs/standrewsglenwood
...extracts nothing.
I've tried an individual file in that dir with no success.
I've tried other files and dirs with no success.
??
Glenn.