NFS export directory not filesystem?

[Lyle Seaman]

I have an Sun running SunOS 3.4  and  several SCO Unix V/386 boxes.
I have no problem exporting filesystems from any of these machines and
mounting them on any other machine, but if I try to export only a 
subirectory (a child of a filesystem root) I have no success.  
For example:
when my exports file contains
/usr

And I do a showmount -e thathost  from another machine I see /usr
is exported.  I can mount /usr on another machine as well.  But if 
my exports file contains:
/usr/src

Which is not a filesystem itself (Note that /usr/src is the only 
thing in the exports file), when I do a showmount -e thathost 
nothing appears to be exported.  

Sun support wasn't much help - they said it "works on a 4.0 machine
configured as a server".

Please e-mail.
Thanks,
Lyle.
comm.wang.com!lws at uunet.uu.net