To narrow it down a  bit, these are inarticulate brachipods (Class
Inarticulata), and I believe these specimens may represent the family
Discinidae, which have a chitinous shell, incompletely mineralized with
phosphate salts.  If you want a positive ID on these specimens, I would
recommend that you contact Ron Voskuil.  His e-mail is [log in to unmask]

        Paul M.