Citing same-author-and-date papers

[Mike Taylor <mike@indexdata.com>]

A boring question about putting a manuscript together ...

Suppose I need to cite two papers by Hatcher, both from 1903 -- first,
I mention that in one of the papers, he considered Astrodon and
Pleurocoelus synonymous, then later I mention the genus
Haplocanthosaurus and so have to give the citation for that name.  The
two papers, in the order that I mentioned them, are:

        Hatcher, J. B.  1903.  Osteology of _Haplocanthosaurus_ with
        description of a new species, and remarks on the probable
        habits of the Sauropoda and the age and origin of the
        Atlantosaurus beds.  Memoirs of the Carnegie Museum, 2: 1-72
        and plates I-V.

        Hatcher, J. B.  1903.  A new name for the dinosaur
        _Haplocanthus_ Hatcher. Proceedings of the Biological Society
        of Washington 16: 100.

The first one I cited is a monograph describing in detail the dinosaur
named in the very brief second one; so clearly they were published in
the opposite order.

My question: which should I designate Hatcher 1903a and which Hatcher
1903b?  Should I call the monograph 1903a because I mentioned that one
first, or should one-pager be 1903a because it was published first?

Thanks,

 _/|_    ___________________________________________________________________
/o ) \/  Mike Taylor    <mike@indexdata.com>    http://www.miketaylor.org.uk
)_v__/\  "By filing this bug report you have challenged the honor of
         my family.  PREPARE TO DIE!" -- Klingon Programming Mantra