Leiopteria is one of the many common Pelcypods found in the Mahantango. It's shell is rounded with a very long hinge line that gives it a look like it has wings. Leiopteria primarily existed from the Silurian to the middle Carboniferous, but seems to be most prolific in the middle to upper Devonian.
Here is an internal mold (front and back) of a shell.
Another internal impression preserved in rock.
Leiopteria belonged to the extinct family Pterineidae within the order Pterineidae. According to Wikipedia, modern families of the order Pterineidae are important economically since they are cultivated as pearl oysters (although they are not at all related to true oysters nor to pearl bearing freshwater mussels). Below is a modern Akoya pearl oyster from Japan. You can see that it has a similar body plan with "wings" along the hinge line.
Some other genera from the Pterineidae that are also found in the Mahantango are:
Leptodesma (with a Leiopteria above it)
The Deep Blue server at the University of Michigan has a great paper regarding Leiopteria with some excellent plates illustrating specimens from the Arkona region.