Empty shells of Horse Mussels (Modiolus modiolus) are commonly found on southern Maine beaches, often attached to washed ashore seaweeds (including Irish Moss and Atlantic Kombu). Shells are typically black on the outside and white on the inside, but wear and staining result in a diversity of appearances. Technical terms aside, notice the characteristic double-bump at the narrow end of a Horse Mussel shell -- Blue Mussels (Mytilus edulis) lack this feature.
Considered inedible to humans, Horse Mussels are eaten by many other coastal creatures, including Herring Gulls who can be seen dropping them repeatedly from the air in an effort to crack the mussels open. I recently watched a Herring Gull crack one open in this way and consume the flesh in short order. (To view the following images in full-size, click here.)