Another very common Brachiopod from the Middle Devonian is Athyris. In the Mahantango formation it's very common in the Montebello member. This member is a fairly near shore series of upwards fining (grain size decreasing as you go up the stratigraphic column) rocks. I'll discuss this more in another post. A good exposure of the Montebello member is at an old quarry on the east side of the Susquehanna, north of Harrisburg, PA near a small town called Ft. Hunter. In some isolated beds the conditions were right to allow abundant life and Athyris is a major player.
This is a typical specimen that is complete and has some recrystallized shell still remaining. The last picture show the exterior shell cast contained in the shale matrix.
One of the most interesting occurances at the Ft. Hunter site is that some of the Athyris have their Lophophore supports (called the brachidium) preserved. This is somewhat uncommon to find as they are fairly delicate, spiral shaped structures. To be able to find a specimen with these structures presevred indicates calm conditions and very rapid burial before the two valves can separate after death.
Below are some examples of specimens I've found from the site with the Spiralia preserved.
From another part of the Mahantango, the Sherman Creek member, comes these two internal molds of Athyris. Notice the variations between the molds which could indicate different species.
For more info on the internal anatomy of Brachiopods (it's more than I can post about) check out this page from SUNY Courtland
Mike over at Louisville Fossils recently posted about this same Brachiopod with images of fossils from his region and some that I sent him from upstate NY. I'm cheating with that last link as I haven't taken pics of any specimens I found from that site for myself yet.