Another very large Brachiopod that I've acquired in estate sales is Derbyia. This member of the Orthotetida order lived primarily from the Carboniferous to the Permian. This particular specimen comes from the Wolfcamp series of Kansas. The overall look of the shell is rounded with an almost globular shape. Other than the large size and plentiful thin ribs the most notable feature is the back of the shell where the Pedicle and Brachial valves meet.
Here you can see a narrow, flat triangular shaped interarea which is unsual for brachiopods and there is not a clear pedicle exit hole at the hinge line. The pedicle was a muscle or series of threads that extended out of a hole in the pedicle valve and enabled the Brachiopod to attach itself to a surface. The current theory is that the pedicle threads extended through many small holes at the very tip of the pedicle valve. This would explain the deep interarea that would expand and get wider at the hinge as the animals shell grew.