Samba Share: smb doesn't set group id bit

Question

I have some problems with a smb share. Every subfolder should get the group id bit. If I create a directory local on the machine with:

$ mkdir local

the bit is set, but if i create it via the smb share it is not set. now it looks like this:

$ ll
    total 8
    drwxrwsr-x 2 pat skynet 4096 Jul 23 23:03 local
    drwxrwxr-x 2 pat skynet 4096 Jul 23 23:03 smb

here is the part from the smb.conf where is set the share:

[pub]  
        comment = pub share
        path = /mnt/pub
        public = no
        browseable = yes
        writeable = yes
        valid users = +skynet
        directory mask = 775

any ideas?