If the structures are indeed biogenic and not concretions (hard to tell from the photos), then they are almost certainly inoceramid bivalves.  I have seen inoceramids of about that size from the Cretaceous Pierre Shale in Colorado.  Inoceramids are traditionally considered close relatives of Isognomon, Pteria, etc. and range from the Permian to the upper (but apparently not uppermost) Cretaceous.  However, Paul Johnston and coauthors have recently suggested that they represent a distinctive extinct subclass, related to the Paleozoic "cryptodonts" like Praecardia, Slava, etc.  They typically occur in dark shales (which seemed plausible from the photos).  The hinges are edentulous, with numerous ligament pits like Isognomon.  The shells often have broad concentric ribbing.  The ligament is somewhat unusual.  The outer shell layer consists of large calcite prisms that are distinctive in the sediment even after the shell has crumbled away.  Inoceramids and their kin may have been chemosymbiotic.  A specimen with possible traces of gill supports suggests a somewhat peculiar anatomy.

George's web page mentions Plagiostoma gigantissima as an apparent guess by the discoverer.  This is a European Jurassic limid (file clam), much smaller than these forms, and lacks the concentric sculpture of these specimens.

The largest known brachiopod is indeed much smaller than these.

    Dr. David Campbell
    "Old Seashells"
    Biology Department
    Saint Mary's College of Maryland
    18952 E. Fisher Road
    St. Mary's City, MD  20686-3001 USA
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"Mollusks murmured 'Morning!'.  And salmon chanted 'Evening!'."-Frank Muir, Oh My Word!