Chonetid type brachiopods are fairly common in the Mahantango formation on Pennsylvania but a recent find deserves a little more notice. It's a shell with small spines preserved coming off the posterior hinge. Only three are visible, the yellow iron staining helps set them off, but this is a somewhat unusual occurance based on my experience. Most all the fossils found in the Mahantango are the results of storm deposits, very few are "smothered bottom" type. As such delicate details like the spines are often lost when the original shell is tossed in the waves.
I collected the above specimen was collected near Seven Stars, PA from the Mahantango formation (Devonian, Givetian stage).