21204.  (a) No person operating a bicycle upon a highway shall ride
other than upon or astride a permanent and regular seat attached
thereto.
   (b) No operator shall allow a person riding as a passenger, and no
person shall ride as a passenger, on a bicycle upon a highway other
than upon or astride a separate seat attached thereto.  If the
passenger is four years of age or younger, or weighs 40 pounds or
less, the seat shall have adequate provision for retaining the
passenger in place and for protecting the passenger from the moving
parts of the bicycle.