>If you had a single lump of grey-blue clay from the St. Mary's formation in Maryland, and it had the top of an Ecphora quadricostata (Maryland State Shell) poking provocatively out of one side, which method would you use to try to extricate it?<

For a large specimen from the St. Mary's and similar deposits, I would suggest getting an old toothbrush and a bucket of water and scrubbing away; a dental pick may be helpful for removing excess sediment.  Disaggregation is primarily if you are looking for small things in a block of sediment.

Incidentally, E. quadricostata turns out to be a misidentification (based on erroneous locality data).  True quadricostata is the form in the Pliocene Yorktown and equivalents, which is largely absent from Maryland.  E. gardnerae is one of the similar species from the Maryland Miocene.  Of course, if you believe one particular approach to Ecphora, you can just create a new species name (and maybe a new genus) for your specimen...

    Dr. David Campbell
    Old Seashells
    University of Alabama
    Biodiversity & Systematics
    Dept. Biological Sciences
    Box 870345
    Tuscaloosa, AL  35487-0345 USA
    [log in to unmask]

That is Uncle Joe, taken in the masonic regalia of a Grand Exalted Periwinkle of the Mystic Order of Whelks-P.G. Wodehouse, Romance at Droitgate Spa